They don't show too much of other dimensions though either, and Mewni is kind of a shithole in S1 (that's why it's appealing from the audience perspective, but I guess in-universe Marco's not exactly seeing it as the best place ever). Actually that's what I liked in the early days of the show, that all the dimensions just seemed to be kind of dangerous hellholes that wouldn't be super fun for Marco to experience.
The difference for me is that the other dimensions at least have some novelty through their characters (Moon, Ludo, Glossaryck, Toffee, MHC, etc. are all more interesting than any non-Marco Earth character/maybe Janna on a good day, because they actually impact the plot) or have some novelty like the Plains of Time. Actually Plains of Time is like the only place besides Mewni that takes the "other dimension" concept to interesting places as well; I can't say I particularly care about Pixtopia or the Bounce Lounge either. Difference is that we're not in those places the majority of the time.
I just can't really think of too much that could be done with Janna/Jackie to make them relevant beyond the roles they already have. Jackie will obviously get some sort of development/exploration when Jarco ends, which is at least plot-relevant, but I'm not sure what else she could contribute. Janna is great and plays a good role as an Earth friend for Star who's not Marco, but she pretty much is just comedic and I think that's fine. Maybe when things get more intense she could try to defend Star, or use her skills at getting personal information to solve a problem, or maybe feel useless as a human or something.
But then that requires her to be in Mewni anyway, so I can't picture a Janna character development episode that takes place before that. StarFan13 is too much of a joke to get a full episode.
They could always give them Earth problems, but even as I like them I don't think I'd care much about their Earth problems as the Mewni stuff heats up.