In the film Young Adult, Mavis and Matt discuss her cousin, who is in a wheelchair. She mentions that the car accident that disabled her cousin happened on the weekend of her sweet 16. Given her self-centered personality, it seems likely to me that as a fresh driver, she was involved in the accident, but doesn't feel any remorse for her actions. Is this actually the case, was she involved in the car accident?
There is nothing to give a definite yes/no answer to this question but based on the conversation I have to conclude that she wasn't involved
Matt Freehauf: When did he get that chair? Sophomore year?
Mavis Gary: Junior. Car wreck. God, he got so much attention.
Matt Freehauf: Yeah, he was the popular cripple. Practically ruined high school for me.
Mavis Gary: Please. That dick ruined my sweet 16. Same weekend.
Matt Freehauf: You are a piece of work.
Mavis Gary: You're a piece of shit.
Focusing on the way she says, "Please. That dick ruined my sweet 16. Same weekend.", she is focusing on the attention that was taken away from her and given to him. She was the popular girl and she is bitter that one of the most important days in a girl's life was shadowed by someone else.
Breaking down the statement, it implies that it was his fault. "Same weekend" as in "the same weekend I was going to have my sweet 16" implies that she was not involved.
It isn't that she has no remorse, though she obviously doesn't care, but that she is bitter simply because her spotlight was taken away.
I believe that Mike and his back story are simply to give contrast to Mavis and her attitude and to show just how deep it runs.