Well, let's put things into perspective: Mia was a teenager. Yeah, she had sex and keep Isabella, but that does not erase the fact she'll exhibit some teenage habits. I don't think she was a terrible mother. She broke up with Lucas after it was clear he was not fit to be in Isabella's life or was remotely interested in being a dad. I don't think she began ignoring Isabella; the writers just neglected Isabella when Mia began pursuing her modelling career in season 8.

That season wasn't the strongest, as the writing was flaky. Isabella became a background character because the writers already established this: Mia was a teen mom and there were little to no plots to integrate Isabella. I don't think she became self-centered. Mia became short-sighted on her goals and looked at what worked in the moment. Also, she's young and sometimes young people look for thrills rather than think about the future. It was clear Mia wanted to make money to support herself more, and part of that was to support Isabella so she could have a good life. I don't think she was a terrible mother, just short-sighted. I'm sure once she grew older and Isabella did too, she would have concrete goals and achieve them.