I don't know, I think all politicians is fair. Unless they're like, SUPER retired from politics, and they're doing the show to promote their charity that finds shoes for disadvantaged children with two different sized feet or something.
I can't think of a politician I would want on the show, even one I liked. (If I like them, I want them to be doing important things, not this!) I think a lot of times, the complaint "don't talk about politics here" is used to silence marginalized voices, but in this context, it's already enforced. So you end up with this person whose mere existence is polarizing, you're not allowed to talk about the real, human cost reasons why you feel so strongly about it, and it devolves into the shittiest version of no substance, red team/blue team tribal politics.
People who are famous for something else, but also have political views, I think are fair game. But when the show casts a purely political figure and then edits out all the politics to try to make them seem like your favorite problematic uncle, that's when it feels like they're just wallpapering over the elephant in the room.
There's certain other "controversial" contestants it feels like this for too, like Paula Deen, but it's almost always true for politicians. Maybe the rule of thumb is, if they are famous for being a "controversial" figure, you have to be able to talk about WHY they're controversial. If you can't, no casting. The flip side of it, in the "good controversy" category, is if they cast someone like Colin Kaepernick, and then he wasn't allowed to talk about WHY what he's doing is so important to him, I would be infinitely frustrated.