What she was trying to do was tell him that she doesn't care if his performance is not good she still wants to do it with him.
This, absolutely, and as I said in the comments to the previous chapter, I'm guessing there was a translation issue or something based on how
he reacted to what she said.
I feel like Azusa goofed last chapter with what she said to him, but this chapter and previous chapters she's been very mature, calm, patient, reasonable... I think she does have feelings for him, and she's handled everything that's happened really well, even after being rejected. I'm not sure if the author is trying to make it look like she's detached and doesn't care that much, but it seems to me like she's very mature.
She's being very mature and sensible about all this, as well as
very understanding and considerate - she could have just walked away at several points and any reasonable person would have nodded sagely and thought "Yup, he's too much of a fixer-upper", but she's persisted and been very careful and gentle about it. And honestly, it's worked - MC is still a mess, but he's so much better than he was. If crazy stalker fangirl wasn't there to mess things up I'd expect their relationship to progress pretty smoothly from here.
One thing I noticed while re-reading the previous chapter before comenting: when she gave him that little peck on the cheek before leaving in the morning, she was blushing just a little. Given the circumstances, particularly the fact that MC was still sound asleep (and looking all cute and vulnerable without his glasses), and the letter she left him, it's really hard to read this as anything other than her being genuine about liking him.
You can see her put the pieces together in this chapter. That Ametella is real, and what Yuuta said wasn't a lie. She even notices that Ametella's super into him, so he wasn't actually lying about that when Yuuta brought it up to make her jealous. She takes it all in stride. She even just eats the accusation that she doesn't actually love him by saying that she didn't realize she came across as such a bad person. Saying that she didn't know Yuuta had such a cute girlfriend. Because, well, she knows he doesn't. This is a setup to make Ametella admit that she's not his girlfriend, and she uses that to deliver the line that you shouldn't try to tie up someone you care about. Something she knows applies just as much to herself.
I've thought for a while that she's being set up by the author to play on the audience's anxieties and self-confidence issues, and that the sense of her as a creepy yandere stalkerish person is a Rorschach test for the reader and how much they identify with all the worst aspects of the MC, and how she handled this situation really reinforces that. Although nothing that she's said in this chapter is clearly threatening it can all be read in multiple ways, and some of those readings can be taken as slightly creepy threats (and particularly creepy in the eyes of someone who's not good at reading non-verbal cues, which is likely the case for most of the expected audience for this manga, as well as for Amatella).
An interesting aspect of her reaction to Amatella admitting she wasn't his girlfriend was that she responded with (paraphrased) "you obviously care about him a lot, maybe you shouldn't be trying to interfere in his pursuit of happiness". This can be read in multiple ways - it could be seen as a subtle "back off", or a slightly self-deprecating "I guess we're in the same boat", or even a subtle challenge to Amatella to carefully consider what it means when she says she cares about him. I actually think that last one is probably closest to what she meant, though it's hard to be confident in saying that - we get absolutely zero insight into what's going on inside Azusa's head, after all.
She could
also have responded with something like "but you want to be his girlfriend, don't you?", which has an obvious answer (though perhaps complicated by the weirdness that is the whole otaku/oshi relationship thing). If she did, the natural follow up would be for Azusa to wish her rival the best of luck in their competition for Yuuta . . . which could also be read in at least five different ways . . .
If I'm right about what Azusa intended to convey, it's notable in that it's not actually telling her to back off or saying
anything at all about what
Azusa's intentions or thoughts are about the whole thing. Instead, it's almost like she's guiding Amatella towards properly considering her own feelings about Yuuta - if she cares about him, she should back off and let him live his life, if she has romantic feelings for him she should consider how she wants to act on them. I can't help seeing that as a kind of careful parental-style guidance of a child's thoughts down a path that will (hopefully) help them grow as a person - which is definitely something Amatella could use some help with . . .
But, well... Ametella is there, in tears. She's going to make him explain himself, she's going to ruin this chance that he's steeled himself to take. Something that wouldn't have happened if he was honest with her about how he felt, instead of leading her on to think that she was someone he was romantically interested in over Azuha. But now, she's going to ruin this chance, what he thinks might be his only chance, that he just steeled himself to take. No matter what happens, he's going to resent this.
I think this is probably where we'll find out if Amatella really
is crazy stalker fangirl, or just the same kind of socially anxious and isolated otaku that the author is having fun subtly poking holes in.
If she really is the yandere character in all this then she'll continue her emotional manipulation, and it'll be clear that she's doing it
consciously. Exactly what happens after that I don't know, but it'll probably depend on how honest Yuuta can be with Azusa about what's going on - I get the feeling that Azusa is quite capable of kerb-stomping a yandere stalker who's threatening someone she cares about, though it's a bit hard to tell whether that's just a hangover from the implicitly creepy way she's been portrayed all along. If Yuuta can explain what's happening and ask Azusa for help then it'll end fairly well for them (though not so well for crazy stalker fangirl Amatella).
Of course, that requires Yuuta to actually
recognise that she's being a crazy stalker fangirl, and have the confidence to talk to Azusa about his relationship with another woman . . . which would take the story down another interesting and fumble-filled path . . .
There are basically two non-yandere options: she decides she wants to pursue him romantically; or she decides she wants to back off and just be a (non-crazy-stalker) fangirl and maybe friend. Option one she gets kerb-stomped in a different way by Azusa (because Azusa and Yuuta have some serious chemistry, despite all the issues); option two may lead to an actual healthy relationship between Amatella and Yuuta, which would be kind of nice to see (for both of them).
Either way I expect to see lots more tears and so forth, because Amatella is still a 19 year old with very limited emotional skills - she was deeply moved and influenced by Dark Knight sensei's middle-school angst-fest, her judgement about any of the subtleties of real-world relationships is going to be even worse than Yuuta's.
I'm not going to hazard a guess as to how this will all pan out - it's obvious this author has a Thing(tm) for crazy yandere shennanigans, but if I'm not totally misreading this story I'm inclined to think they're playing games with the tropes and their audience rather than indulging their weird fetish . . .