Honestly this isn't enough to justify it being in anything other than E. You couldn't even argue that this is enough of a niche to be D tier really, since plenty of mons can cripple stuff with status, and as TropiOUs pointed out, there are plenty of pokemon that do that better than Regigigas as well.
I don't know what tier Regigigas should be in, I've never used it before.. Just 99% of the time whenever I see people talking about it, they're talking about trying to sweep with it. Which is wrong in my opinion, that's surely almost impossible to pull off in a real match. Regigigas should be used as a defensive/support Pokemon, not as a "somehow survive for 5 turns and then sweep" Pokemon like everyone seems to suggest.