There are other factors too like there being less to bicker about in the summer. I don't know about your kids but there isn't much they have to do that they don't WANT to do in the summer. They go to bed late, they sleep in. When they have to wake early it's for It's for an activity they are passionate about. Both my kids love school in general but there will always be aspects they rather pass on... in the summer, there is almost nothing they'd want to miss. 12 is an age where girls in particular can be difficult and pushing for more control. During the school year, they just have less control and it's not be your fault at all but your the one she knows will love her even when she's nasty. In the summer, they have less to rebel against.
Is she an introvert naturally? If so, introverts get their energy from solitude. As much as she enjoys the other kids in school, they may also be zapping her of all her energy. If last year was you DD's first year in school, it's very likely that she hasn't striken a balance yet. She HAS to keep it together in school and so when she comes home, you get the over-exhausted meltdown. It took my DD a couple years to really find that balance (but she was less volitile at home because she was 5 and 6.) She had to learn how to carve time for herself during the day AND at the same time increase her tolerance for delayed solitude. She got really good at it. Even then, this first year in high school was tough because she went to a specialty school with an entire campus full of kids who shared her interests. Instead of spending her lunches with a favorite teacher, a couple quieter friends or taking a walk on the field like she used to, she's hanging out in large caotic groups for 6 hours staight. Yes, she's having a blast but comes home and locks herself in her room for 30 minutes and I don't argue. She needs that time to recharge before dealing with the family. We ALL need her to take that time lol.
Enjoy the summer and take advantage of this time to reconnect and have fun with your DD. Hopefully, it'll continue. I know DD and I went through a rough patch this past school year too (she's 14 but just started her period so hormonally like many 12-year-olds.) It was bad for about 6 months and then it started to get better. We're approaching normal again and it makes me happy!