I respectfully disagree. The restaurant is bad enough. Actually its awful. She's awful.....
And the picnic? She didn’t dump the mc because he turned down her picnic. Turning down her picnic meant a series of events that would have further deepen their relationship did not take place. That’s not the same thing as "You chose Sage over me, you’re dumped!"
And if Jill didn’t feel the relationship was right for her, totally legit for her to break it off.
My biggest gripe with her is that when she does break it off with the mc, she does it publicly in a restaurant. Tybalt deserved it, and if the mc had been fucking other girls left and right, then he also deserved it, but the loyal, blue-balled mc, he didn’t deserve that. A more private breakup would have been better, like when the mc dumps her on the "dumping bench" (not to be confused with all the other benches in the game).
Still, none of that makes her "VERY bad".
...
But Fuckface literally TOLD her Jill had to take care of a friend today, when she SURPRISED him with a last minute picnic. If i want to take someone on a secret adventure, and I don't tell them to keep a timeslot open, have zero right to be offended when they can't do it. This is Jill's fault, but she (and the game) treat it as if Fuckface did something wrong.
So with that in mind, Jill is either stupid, or awful. Either she can't get her shit together enough to send a text saying, 'hey, keep your afternoon free', or she's petty enough that she blames her own fuck up on someone who is an objectively 'better' person than her.
The most important quality in any relationship is 'is this person a good friend?'. Are they there for the people they care about? Can they put their own needs second and seek the other person's happiness?
Does Jill not recognize this? Does she not care? Again, if its the former, she's an idiot (partially excused by the fact that she's an adult virgin, but still). If it's the latter, she's the kind of person who's morality is 100% centered around themselves, ie. things are 'right' and 'wrong' based on how they effect ME.
So yeah. Jill is stupid or she's awful, or she's both.