I see a lot more criticizing people for wanting RPG, Warioware etc characters than I’ve seen any real belief of them doing so. Clearly they're a staple of dream rosters but I don't think anyone's expectations are warped about who usually shows up in these games. Getting to the point where I've seen a few people just be outright mean about it - not necessarily here, just in general.
In any case, in respect to Warioware, these worlds usually don't overlap (except once, in the Mario Kart arcade game's Diamond City track). The reason why characters like Ashley and Mona are unlikely to appear in Mario Kart are probably comparable to the reasons Waluigi doesn't appear in Warioware. Wario is a vessel to explore these brand new characters made by Ko Takeuchi, who have virtually nothing to do with Mario otherwise. I would personally like to see them make the jump over, but I think this series has established its own independent identity that it seems to want to keep.
I think though, had Wario Land been a bit more fortunate we could have seen Captain Syrup or someone. Wario Land has had a passive influence on a number of spinoffs already, with the theme from that game being present in Mario Tennis and his shoulder bash appearing in other games like Mario Strikers. Being a platforming series much like DKC and Yoshi's Island, I think it may be easier for them to visualize as an extension of Mario's universe - Wario Land 1 is called Super Mario Land 3, after all. I'm not holding my breath for it with MK9, but I think in a different world Wario Land would be in similar standing to Yoshi's Island.