Jill isn't after a "bad boy" who gets into fights and starts trouble, so she's disappointed that the MC is getting into trouble (first Dawe and the wedgie, then Chad, now Tybalt), but she still likes the mc a lot (possible Florence Nightingale effect - or maybe it's because she's already seen his cock
).
Plus, she (naively) considers Tybalt as a friends, so she doesn't want to see him beat up.
But DPC made the scene where the mc punches Tybalt so satisfying (I mean he even added a replay button - it's like he's actively condoning it), so I don't think that one action is going to fuck everything up.
And the reason why Jill doesn't want to see the MC anymore is that Tybalt has indirectly blackmailed her, saying his friend may press charges against the mc if she persists in seeing him. Don't worry, that shit will be sorted soon.