I'll leave a list of what does work, what doesn't work (and should), and what theoretically could (but probably shouldn't) work, but won't presently work.

Works:
/mw create multiverse:<worldname> NORMAL
(Yes, there's a leading space after NORMAL. Don't ask me why.)
Doesn't work, but should:
/mw create multiverse:<worldname> NORMAL
(without the leading space, it doesn't work.)
/mw create <worldname> NORMAL
(I know the interpreter inserts "multiverse: if you forget to add it, after reviewing the code, but this still bronks.)
Could work in theory, but can't right now because ???:
/mw create pixelmon:<worldname> NORMAL
(I'm not sure what's excluding other types of worlds/dimensions from being generated, but unless it's specifically multiverse:, or vanilla, it returns "null")

I'd make comments directly in the codebase to suggest fixes/updates, but I don't have a convenient way to fork/rebuild the plugin yet. More testing to be done on my side anyways.