Well, there's three kinds of mixing:
1- marketing/organizational which can lead to one picking up or buying a book expecting something entirely different. Bad, very bad.
2- confused writing and/or editing. Stories that start out as one and meander into the other or that just can't make up their mind. That's just bad writing.
3- Then there's intentional genre mixing. For, Larry Niven's Svetz stories (Flight of the horse, etc) proposes, quite reasonably, that if Time Travel is impossible by known physics, then any time travel story is by definition fantasy. Of course, his time traveller doesn't know this. So his "past-time" encounters with witches and unicorns, ghosts and assorted monsters keep biting him in the rear.
Me, I tend to think of crossovers/mashes as outright fantasy. Period. Want to pit vampires against alien invaders (as David Weber tried and failed to pull off?) no problem. Want to mix space adventurers against alien vampires (as C.L. Moore's Shambleau successfully did?) fine by me.
Each genre has its conventions and its rules. Good writers honor them.
In fantasy anything goes; sparkly vamps or blood-curdling Lovecraftian horrors. I'll cheerfully buy anything that's well-enough written. I don't mind SF elements in my fantasy. Just don't put fantasy elements into a story and try to pass it as SF. Because that suggests you don't know what SF is or you don't know what is science and what is urban legend. And at that point it just becomes bad writing.