Ness is a character that is going to be strong early because his common/traditionally bad match-ups aren't prominent yet. Once characters like Marth, Lucina, etc (characters generally with more range than him) become more prominent, Ness will fall down. He doesn't have many tools and while the tools he does have are strong they don't make up for his usual problems that they've still yet to fix (slow ground speed, PK Fire isn't a good projectile, susceptible to pressure, terrible range on the ground, etc etc).
He was seen as pretty decent at first for Brawl as well, if I recall. Then the whole grab release infinites sent him way down + once he was figured out in general he became terrible lol.