Though I can say that if Spidy was a mutant, he'd lose his strength and agility too.
Null's only limitation is physical mutations. Colossus, Cable, and Beast are all jacked regardless of their powers.
There's also the fact that people with more skill than him can beat him. Psylocke, Wolverine, and Laura all know multiple martial arts.
So he can stop Nightcrawler from teleporting away, but Kurt doesn't forget his fighting skills.
This is why Xavier and Storm have X-23 teach him how to fight. Null was already considered medically fit and athletic Nulls may already be trained prior to the game starting.
Also Null can't nullify Newton's laws of motion.
An example shown in game.
He can stop Jean from floating tennis balls toward him but if she chucks them them with her powers, they're still going to hit him because it's a thrown tennis ball.