I feel like you're looking way too much into this because most of the known routes don't really have a fitting boss.
Isabelle's route is all about fighting other female characters but there are currently no female bosses.
Huhhh, why does it make my theory make no sense? We don't know all the bosses right now, there could be a female bosses for all we know
Also, the boss we have seen have make senses with the theme they have:
Mario? Bowser is Mario nemesis.
Wolf? Galleom because the theme is character that they were cut between games.
ROB? Galleom because he is a giant robot and because his expression never change.
Duck Hunt? Animals, Rathalos is a animal, sort of & Monster Hunter/Duck Hunt
Cap.Falcon? His theme is character who fight up close, Bowser does just that.
There is a theme for each characters, and the boss they have always follow more or less that theme, so I don't see how I'm reading too much into it.
Also, for Dracula, you probably be able to damage him only if you hit his head, just like in his game.