Thus far I haven't found any monster mash earlier than the 1933 cartoon MICKEY'S GALA PREMIERE, although to be sure this is what I would call an "incidental mash." Mickey goes to a film premiere, where a lot of Hollywood celebs are parodied, including three movie-monsters of the period: Dracula, the Monster, and Mister Hyde. They're only on screen for less than a minute, so if you want a "mash" that has *substance,* you've got to go with 1934's "Waxworks." ("Incidentally," another "incidental mash" I'll probably never devote a post to is the Mighty Mouse cartoon "Bad Bill Bunion," in which Dracula and the Monster make tossed-off cameos.)
Offhand I don't know if any other Disney productions used monster-mashes, but 2021 debuted one that qualifies as more than incidental: A TALE OF TWO WITCHES. It's mostly about two lady witches, Minnie and Daisy, learning magic from Mistress Clarabell, and fighting a spectre named "Pete the Ghost." Mickey, Pluto, Donald and Goofy all appear in monster-forms but have very little to do in the story-- though at least they're around for more than simple cameos.
