You want an analysis? I'll give you one.
I don't oppose it because Janna's a gypsy, nut because she's dreadful and delights in the pain of others.
That makes her such an amusing character; being so unapologetically amoral. It's the core of who she is, the sole thing making her interesting and fun, the reason people remember her at all. She messes with people and doesn't respect rules or boundaries.
However, her character core also makes her a terrible match for Tom. He's canonically got huge mental health and anger issues. He's presented as a vulnerable character who's struggling to improve himself, not as a joke. The crew seem to like him. He's bad at dealing with people not doing as he wants, heck even Star clearly winds him up. The one person who brings out the best in him so far has been Marco.
So, seeing how he can't even deal with Star, he absolutely couldn't deal with Janna. Janna's all about provoking and needling other people for fun, and combined with Tom's explosive anger issues it'd lead to him killing her or at least having all his aspirations of self-control and self development annihilated for Janna's amusement. That'd be tragic, and I don't think the crew want to treat Tom like that.
The only way to make them a compatible match would be to rewrite both into totally different people. Especially Janna, whose only characteristic is "smug asshole thief who hangs with Star", would need to be heel-turned into a totally different person. But she's just a Brittney-tier background character who's only gotten the screentime that she has because the crew liked her smug assholery - so rewriting her into someone suitable for Tom would also remove the reason she ever got more screentime than Alfonzo in the first place.
I just don't think the crew cares about Janna enough to wreck Tom's development for her, nor do they want to ruin her character by rewriting her into someone chill enough for Tom. So, it's a shit ship.