Idk why everyone is so angry with Yuuga
He's just dense as a black hole, and it's okay. It's like people can't have faults

There are a lot of people like this, that's why if you love someone - you should tell them that, and not just try to appeal to them or just be kind to them and hope they understand your feelings

So I'd say that Mikana is also at fault for not actually conveying her feelings properly
Let's also not forget that Mikana is, at her core, a tsundere, and as we all know, when a tsundere says or does one thing to her love interest, it can come off as something else entirely, mostly for the worst. She's also Yuuga's childhood friend, so that's a double-whammy, and add into how she encouraged leaving the Yuuga's previous crush (Cecilia) to die at the beginning, souring their friendship even further, and that's a triple-threat of losing-heroine obstacles Mikana laid out for herself. Oh, and they're all teenagers, if I remember correctly (except Raven, I think?), so chalk puberty and hormones on that list, too.
Sure, the abandonment thing has been smoothed out and Yuuga is quick to forgive, it still leaves a stain on Mikana's conscience, making her further hesitant on whether she should confess her feelings to him (which, from what I can remember, we have
not seen that happen yet). From that point on, though, she has just been 'making efforts to be noticed', which looked to have
finally worked, but it did
not turn out the way Mikana had hoped.
We can all agree that Yuuga is as dense as a black hole, but even
she knew that, and Yuuga finally coming to that realization of her feelings for him the way he did? Had also given her a revelation of 'what the fuck have I been
doing all this time?' and
not just confessed her feelings to him
waaaaaay earlier.
So yeah, in short, both of them are so stupid that they're practically made for each other, much to everyone's frustrations.