Diane can't deal with the fact that she's a bad person herself, se usually surrounds herself with people she thinks are lesser (ie, Mr. PB is dumber than her so she feels smart, Bojack is a shit person so it makes her feel validated about her morality) but she always cracks when she's not "the best", she cracked when Irving was "better at feminism", she cracked when St. Clair was "better at activism" and she cracks every time Mr. PB does these amazing, romantic and sweet gestures, because she is reminded that she doesn't deserved them. She's so addicted to be the moral high ground that she's often willing to hurt her husband's career (on tv or politics) just for that rush of superiority, she basically can't deal with the fact that she might be the problem.
What the fuck? Diane is written realistically and she has her own share of problems and insecurities, but this is very much painting the door black with her character. I genuinely don't remember what happened with Irving or what "better at feminism" is even supposed to mean, and I don't have the time to rewatch the episode right now, so I can't defend that one, but the other examples here are being drastically misrepresented.
Her relationship with Bojack is complicated in that they use each other as emotional crutches, but to imply that she is exploiting Bojack is wrong. They're just there for each other because they relate to each other's depression and insecurities. As a result, they understand each other very well, but outside of the first season where they're getting to know each other, I can't think of a single incident where either Bojack or Diane exploited their relationship with one another for some kind of power play, and most of those were from Bojack's side. When I see them interacting with each other, I don't get the sense that either is vying for a position of superiority. I certainly don't think Bojack would consider his life improved by Diane not being in it.
The other stuff comes off as even more ridiculous to me, especially St. Clair being "Better at activism". St. Clair didn't give a flying fuck about the people he was 'helping'. The visual gags of him are all about him throwing money and resources at people in need without caring whether they can actually use it. The most you can say about Diane is that her desire to help people was motivated by her own insecurities with her marriage and life in general rather than pure altruism. That's not malicious, it's just the struggle of a person who is trying to find themselves. She certainly didn't hurt anyone doing it, like St. Clair routinely does.
And as for Mr. PB, he's a very, very good boy, but that doesn't mean he's the right fit for Diane. People need to stop defending his "amazing, romantic and sweet gestures", because it's ignoring the fundamental fact that those gestures aren't things that make Diane comfortable, and after 3 seasons of her being his wife, he should know that by now. Mr. Peanutbutter isn't malicious, but these gestures are unintentionally selfish. They're things that he's doing to make
himself feel that his relationship is more secure, but it only makes things worse because he's still at a point where he doesn't seem to consider what his wife thinks. That's a necessary aspect of doing something nice for someone: You need to know, for sure, that it's something the person wants. This was especially apparent in his stint in politics. You're suggesting that Diane is being selfish because she doesn't want to unconditionally support Mr. PB as governor when he'd be fucking awful at it? When he's actively wreaking havok in Diane's own life by fracking their backyard, destroying their home, and putting their entire state in jeapody with comedically tragic government misspending? She has no right to say "No" to any of this when none of it is something she wants and it's being forced on her?
And in any case, here's the actual problem: That time is spent trying to figure out who is the worst person at all. The characters here are written with very realistic emotional fault lines. They have vulnerabilities, insecurities, biases, blindspots. The flaws presented in the characters aren't about finding whoever is the shittiest person and then jeering at them because they suck. It's about these people learning to live with these things as they're constantly hurt by one thing and then the next. Life is an ocean in storm around them and they're just trying to hold on to their raft and make it through alive. Even if Diane was 'the worst'...and this reading is seems to be largely based on her not being a feel good happy spot for other characters to enjoy, apparently...all that means is that she'd be the most interesting character to watch. But I'd still say that goes to bojack, who has damaged far more lives in his wake than just about any other main character. Bojack has certainly exploited a far greater number of people for the sense of superiority you're attributing to Diane, with Todd being his most frequent crutch. But these all serve to make Bojack an interesting character to explore. They're positive things, in terms of reading the story. Much like how Diane's flaws are positive things about her as well.