I'm not sure really where Ness goes now. He's not top 5, I think an argument could still be made for top 10/15. He doesn't have very many bad MUs - even in MUs where some characters beat out his neutral, he beats them out anyway in other areas usually
This is the key for Ness this time around- he has very few truly
bad matchups. And even then, none of them are death sentences like half his matchups in Brawl. The problem I suppose is that the few that he does have (I'd say Sonic and Rosalina are bad, Diddy and Sheik are unfavorable. Maybe throw Mario in there as unfavorable, but I feel there are enough tradeoffs and things Ness can do that Mario's FLUDD/Cape don't push the matchup over the edge for him.) are the best, and in effect the most common, characters in the game.
In terms of customs, while I prefer the regular PKT as a juggling tool, Lucas' PKT custom is still a good move for gimping and completely negates most of the cast's ability to gimp him by hitting the head of PKT or suiciding into him to cut PKT2, because a) PKT doesn't go away unless absorbed/reflected b) PKT2 goes right through people so no more suicide>tech to stop him getting back. So at that point, Villager, Rosalina, Fox, and Falco (idk, maybe I'm missing somebody else) are the only real gimp threats, with Rosalina being the only really bad one. PK Flash is still terrible. I can see PK Fire Burst being helpful in matchups like DeDeDe who can just float out of regular PKF very easily or Zelda with Nayru's Love (I think that's what that's called?). Might as well use the pull-in PSI Magnet against characters who don't have anything Ness can absorb.
When you get down to it, PK Fire is honestly not a very good move. It's easily shielded, easily DId out of, and has terrible cooldown. It's not very safe and you'll likely end up grabbed/dash attacked if it doesn't hit. At high levels, it's basically only useful for a punish. Watch FOW and how rarely (if ever) he uses PK Fire (at least from what I've seen).
PK Thunder on the other hand, is fantastic. Makes offstage hell for most characters, or even if your just far enough from Ness that you can't punish him after it ends. I love it and would really miss it if I had to change to Lucas' to prevent gimping.
I think he's top 10, not top 5 (in direct contradiction of my title atm), though. I'd say: Diddy, Sheik, Sonic, Rosalina, and Pikachu are all definitely better than him. Then Zero Suit, Luigi, Fox, and Yoshi in the same relatively-fluid "A/A-" tier as him. He's (finally) a good character, no doubt.