Honestly its moveset holds it back hard enough that even if it had a "neutral" ability like, say, Pressure or Clear Body, it would still be a fringe pick at best.
Mononormal bulky attackers aren't exactly in high demand, only reason Snorlax was used was cause Belly Drum, and Porygon-Z happens to have nuclear boosted stabs while dynamaxed off 135 spatk and adaptability.
Even without the detrimental ability, I have a hard time seeing Regigigas having much place in VGC.
In BSS maaaaybe, but again, it's a maybe, getting pretty much hardwalled by ghost types isn't exactly nice.