Its not outclassed by Volcarona because its presumably going to be in a different tier. And Venomoth and Masquerain have 2 different abilities setting them apart. And Masqerain arguable gets the more useful moves for sweeping. Water/Ice/Bug is better than Poison/Bug/Psychic really.
What sets Masquerain apart from other Bugs with Quiver Dance is that it gets a way of hitting most Steels for neutral damage with Hydro Pump (others can hit neutrally if they have Tinted Lens, and Volcarona like you said will be a different tier). Also, its rare for said Bugs to have access to a 120BP move, as most of their highest BP moves are around 90 or so.
Also, as an aside, having access to Ice type moves might help Masquerain distinguish itself among its peers, but a combo of Bug/ Water/ Ice is walled by quite a lot:
Empoleon, Keldeo, Jellicent, Poliwrath, Qwilfish, Shedinja and Tentacruel.
Personally, I would go for duel STAB and use Bug Buzz along with Air Slash, as well as Hydro Pump. It gets much better coverage, being walled only by Dialga and Empoleon. Plus, the ability to hit the Fighters that Masquerain will no doubt be switching into for SE damage, as well as the Flinch chance, should come in useful.