JOE!
Smash Hero
I was gonna reply to @ Thane of Blue Flames but DMG beat me to it.It's not a weakness if your character can play around instances where the option probably isn't viable. The problem with Ness is that so much power is tossed into PK fire and into grabbing, that it doesn't make sense to hold back on those options and attempt other stuff in Neutral.
Ike has a lot he could do in Neutral besides QD (he has a big meaty sword that loves to hit many different angles). Fox can run around and Laser. Falcon (while fairly linear and limited in his own right) can opt to play decently strong DD/Dash options. Ness does not generally have other things that make as much sense, or that end up being just as effective.
Having a decent MU with Fox is one thing, but he has some struggles. It's very obvious he has a problem with DK, and while he's a tad better at dealing with CCing characters, that's traditionally been a thorn in his side. PM isn't going to be huge on "Ok I got an even MU with Fox, so I'm secured in top tier". PM is going to be about "Can I go up against the general cast, and not have too many bad MU's?".
The general cast is MONSTROUS at handing out very bad MU's back and forth. You really have to watch your back in this game.
Ness isn't fine due to how over-reliant he is on PK fire. The other guys mentioned have other options at their disposal that are just as good & viable as the ones mentioned whereas Ness really doesn't, thus polarization.
Second, when did I ever say Roy is "low tier"? He is simply "Below Average" in my eyes due to again what DMG covered. MU's in general are incredibly important in this game seeing as you can literally run into just about any character (barring 5 or so that I mentioned) and they can be a threat.
Thankfully, it seems that that vast majority of MUs in this game are evenish to the point of player skill being the determining factor of a given match. However, those with more disadvantaged MU's I'd put in "Below Average" (the Heavies all face an uphill battle vs zoners) and those with more good than bad are in "Above Average", and so on. This doesn't mean a BA can't compete with an AA, far from it. It's just that in the grand scheme they aren't as powerful, you dig?