Don't overthink this. In a typical internship, the designated mentor is principally responsible for the intern's work. Your mentor is reviewing your code. You shouldn't be worried about whether she is doing the code reviews correctly, and your judgement in this matter is not important.
It is normal for other seniors to also sit in on code reviews, but they have no obligation to review your every code change, unless their boss has asked them to do so. In the world of business, work priorities keep changing frequently, especially for senior level members. Chasing after every project activity is not feasible. It is regular occurrence for the most important task of one week to be entirely dropped next week (and vice versa).
The most reasonable explanation for Bob and Carla's "neglect" of your code reviews is not that they find your code too terrible to review or that you have offended them. It is simply that they now have more pressing matters to attend to, and they consider it sufficient to trust Alice's judgement.
Moreover, if they truly had reason to believe your code was terrible, they would respond by scrutinizing it more strictly, not by ignoring it entirely. That also suggests (as Wesley Long points out in the comment) that they now consider your code good enough to not require reviews by everyone.
If this still bothers you, you should talk to your mentor or your boss about it. If they see the lack of reviews from Bob and Carla as a problem, they will figure out how to remove the impediments and make the reviews happen.