Even when alive, one is supposed to trust only in Hashem and ask others only as hishtadlus, as we see from yosef hatzadik, that even asking twice was already more than acceptable hishtadlus on his level.
Certainly to beg a person – tzadik or otherwise – would be largely lacking in bitachon.
And when they’re deceased? It’s just ridiculous, because even bederech hateva they can’t help. The only way they can help is if you ascribe powers to them that they do not possess.
Or if you ask them to daven for you, which is perfectly fine.