O BOY, WHAT A QUESTION
The thing is, as long as protag maxes Yukiko’s S. Link, she does become a decent cook by the end. And during Arena, Yukiko’s lunch was actually pretty good until it got messed up by shadow world shenanigans! The biggest hurdles for Yukiko to overcome when it comes to cooking is that 1) she doesn’t know shit about food, 2) she’s convinced that doesn’t matter, and 3) she refuses to ask for help for the actual process of cooking. She probably asks the cooks for advice, but she seems like the type to adamantly refuse practical help in the kitchen - she’ll ask for recipes and instructions, but she has to do EVERYTHING BY HERSELF, FOREVER. I think it’d take a while for her to consistently make edible food, but it’s a process she’d probably work on a lot. A moment of silence for her taste-testers. As long as she gets out of the mentality of trying fancy cooking shit with zero prep & prior knowledge, I’d say she has a low chance of a repeat of Mystery Food X.
As for Chie, I think she wouldn’t put learning to cook as a priority, but I think she’d be a lot faster at learning it - I can see her asking Kanji for help, and actually learning from him (as opposed to Yukiko’s ‘let’s just wing it’ philosophy). The problem is while she’s more inclined to accept practical help, it’d probably take her a while to seek it in the first place due to embarrassment and having to admit to herself she needs it. She is generally more self-aware than Yukiko, though, as long as nobody’s criticizing her. Given that, I think they’d actually be non-lethal cooks by the end of the summer after P4 takes place. Not that there wouldn’t be. Incidents. But at a much lower rate, probably.
Also, they’re probably only non-lethal when cooking separately. The second they’re both in the same kitchen they probably slide back into their Bad Habits.