You can do this without an addon. I use the following macro:
#showtooltip Arboreal Gulper
/run if GetZoneText() == "The Maw" then C_MountJournal.SummonByID(1304) else C_MountJournal.SummonByID(0) end
This will use the Mawsown Soulhunter if you’re in the Maw, and will use summon a random favorite mount if not. Change “Arboreal Gulper” to whichever mount you’d like displayed on the macro’s icon.