I mean, it's pretty clear to me from reading the negative comments in the posts commemorating her death that they took issue with the fact that she wasn't 100% progressive in all respects, that she believed in working with conservative politicians to get things done via compromise, and that she didn't think corporate suits and cops were all evil. In other words, she was a moderate Democrat.

So, from what I can tell, that's the why, but the who really does matter, because you were specifically asking about the comments you're seeing here on Lemmy, and Lemmy is disproportionately representative of Far Left Anarcho-Communists (i.e. whack jobs with a lot of passion, but little intelligence). You wouldn't see these kinds of criticisms of Feinstein in a more balanced political forum as they'd get drowned out by the sensible people.

Edit: Here's a good article detailing her complicated relationship with the gay community, which also sheds some light on why she wasn't the perfect liberal.