I see it too less in the teams. Still it is RU, but I managed to sweep the opponent using it (3 - 0). I had chosen it as my lead, while the opponent took time to set up, it would spam curse two times and then sweep the opponent. So how good is Curse Snorlax in NatDex?
I asked if Snorlax was viable on Smogon. A Smogon user named KirbCena said:
Not really, as it gets halted by stuff like Haze Toxapex, Unaware Clefable, etc., and is too frail on the physical side to comfortably stomach attacks from other common mons like Mega Medicham, Landorus-T, Kartana. etc. as they can boost faster than it and remove it with super effective or SD-boosted moves, especially if they have Z-moves. Rest + Sleep Talk is also unreliable and a huge momentum drain.
Basically, Snorlax isn't really a viable Pokémon in general in Nat Dex since it is really slow and cannot really take physical attacks from common Pokémon in the tier.
Source
Hope this helps.