Genre barriers are not a real thing. Playing with conventions can indeed be fun, but it's writing about writing. Even comic books can be about real emotions and relate to the real world when the writer takes the material seriously (not solemnly! that's not the same thing at all,) rather than playfully.
That said, I repeat, I'm not against fun.
PS Sorry for the accidental double post.
It depends on the work, of course. I'm not going to insist on inserting a leprechaun into a "serious" hard-sf novel, unless you're talking a holodeck or "Shore Leave" planet scenario, but when in doubt I tend to lean toward
playful if you can get away with it. If genre conventions block you from doing something cool and fun and interesting, damn the conventions, full speed ahead. "Playful" is generally a positive in my book.
And, to my mind, a cross-genre story doesn't have to be writing about writing (although I confess I enjoy that kinda thing sometimes). Sticking an ancient Egyptian mummy into a space opera doesn't mean you can't get invested in the characters and their challenges and feelings. An alcoholic starship captain wrestling with the demons of his past can still have "real emotions" regardless of whether he's tracking down a space pirate or a cyborg or a were-coyote.
Or a cyborg were-coyote space pirate.
As for "grotesque" tonal shifts, I gotta admit that one of the things I most enjoyed about writing for DC years ago was that sometimes I got to write a different genre every other scene: I could do spooky horror stuff with the Spectre, Greco-Roman fantasy with the Amazons on Paradise Island, and gritty urban crime stuff in Gotham City . . . all in the same book!
It was a blast.
(That being said, I did once ask an author to delete an extreme gross-out horror scene from a novel just because it felt out of place with the rest of the book, which was more of a romantic fantasy about elves and fairies. But that wasn't a case of genre conventions so much as that one particular scene standing out like a bloody, dismembered thumb.)