Mewnium Z makes Psychic into Genesis Supernova, a base 185 power Psychic type special Z-move that also generates a Psychic Terrain for 5 turns.
It's not worth using a slot for the following reasons:
- Mew is mostly used in a supporting role, so items such as Leftovers or Mental Herb is better.
- The Psychic terrain can also be set by other moves or abilities (Tapu Lele, if it's not banned) which makes this Z-slot and move redundant.
- It takes up a valuable Z-move slot that other Pokemon can make far better use of.
- The time it takes to switch Mew out for some other Pokemon to come in and take advantage isn't worth it, as these Pokemon usually only end up getting 3 turns of the terrain anyway
Given that most Taunt users tend to be Dark type, Mew usually needs to be switched out anyway rather than slapping on Psychic as its attacking move. The only use I see for this z move is in extremely niche circumstances for a gimmicky win at best.