I don't understand the amount of hate Marin's getting this chapter. She's a teenager whose dad scolded her into going to pay a formal visit to people he's never met immediately after learning that she's apparently gotten so close to this family that they regularly feed her and provide her with outfits that are such high quality that celebrities are raving about it on social media. Yeah, she gets to see her crush on New Years, but her dad is there the entire time, nagging her about everything from her manners to her cooking skills, which she obviously thinks are just fine. He obviously doesn't get what she's into and even goes so far as to call her fave's outfit a dress. It's all super embarrassing!! Whether Marin is worried about having being ignored at the con or actually deeply annoyed by it, having to sit through her dad being totally uncool (or worse: being someone who can't appreciate just how incredible her faves are??) a n d making her sound like some kid who can barely look after herself would definitely override everything else she might be thinking about.
I think Marin's dad's reaction was great, personally. He may have been awkward and lame in her eyes but she still talked to him about her cosplaying suddenly blowing up. And her dad could obviously tell that the partnership she and Gojou have going on is a big deal, and he took it seriously. He may not understand why Marin cares about these characters so much but he does understand that she's met someone who cares very deeply about her, or else he wouldn't have made costumes that fit her so perfectly. And I think Gojou's reaction to her dad's request was also perfectly fitting for his age, because even if he acts mature, he's still a kid who hasn't yet processed his feelings for Marin. Of course he's going to politely agree to keep feeding her, what else could she possibly need?
The writing's still good. I don't think Marin's totally out of character. It's just a slow burn romance between two not-so-experienced teenagers. They'll get there.