Eustace Strych was terrible in Billion Dollar Boy but I found him alright in King of Mars and League of Villains. I actually don't remember the JTPH3 appearance much as it was very brief.
One of my least favorite characters is Judy, Jimmy's mom - not herself, but rather the way the writers used her. She had a lot of potential as in the movie and some early episodes like See Jimmy Run they made it clear that she was where Jimmy got his genius IQ from, but in general throughout the series she just functioned as a stereotypical sitcom mother/housewife. Because she's clearly very talented, I think she should've gotten more badass moments like Cindy did later on. This would've made her a better character IMO.
I also hate Nick (partially because Cindy's crush on him feels unnatural) and April and Betty Quilan, again because I don't like their ships (Um? Hello? Jimmy? There's a little someone I'd like you to meet called CINDY! YOUR ONLY REAL CRUSH!)
Nick, though, is one of my fave minor ones, simply cuz he's so funny when he tries so hard to be cool... 
I do kinda wish they'd given some more depth to some minor characters, so you'd know why Nick or Britney are the way they are. I kinda dislike Betty, too, since Jimmy so easily swoons over her and not much was shown about her... She did seem to have a bit of spunk in the ep Vanishing Act, when she separated Jimmy and Cindy when they were squabbling, then pulled her aside. *shrug* She would've been developed more had there been a Season 4, and same with Nick... I do hope they revive the show someday... If they do, that might give me a reason to not dislike Betty so much, and I'd hope they'd flesh out her character more. She's just incredibly underrated and I don't see a reason to like her much... Sure, she's the pretty, popular, nice girl, but we don't get much more than that... 
