DJ sweet spotting isn't really a primary form of recovering for any of the characters with big slow cancelable DJ's (Yoshi, Mewtwo, Peach) so that's not really a unique ness weakness. These characters mostly recover high to recover successfully.
Anyway I'll bow out of this discussion since I don't really know what I'm talking about but all I'm saying is Ness' recovery seems more boned super hard by some characters where as Roy's is consistently bad.
Ness isn't comparable to the other characters without DJ sweetspots because their alternative recoveries are actually good.
Let's compare Mewtwo's recovery and Ness's recovery.
Mewtwo has an invincible teleport. This fully invincible recovery (when active) comes out near instantly and he can sweetspot the ledge with it (which is like DJ sweetspot but you're invincible before you grab the edge and thus are on crack). It also has no landing lag with proper execution (so if they try to get rid of your edge crack, you can land onstage safely; as a bonus is his floatiness allows you TONS of time to see which one they're covering). He also has the most aerial mobility in the game, and is massively floaty. His airdodge makes him disappear. It is very feasible for him, as a result of his aerial mobility and invincible teleport, to jump backwards, go over the opponent, and float to whatever side he wants, teleporting to an appropriate spot if he's approached. Or teleport THROUGH THEM if they attack him as he's trying to float towards the level in any way.
Ness, in comparison, is less floaty, has less aerial mobility, has no invincible Up+B so he's automatically far more vulnerable to any kind of projectile. His priority is in obvious spots so you can space crap during his free fall on him really easily, etc. His Up+B takes a minute to do anything and there's no invincibility. Because of his lack of invincible, lagless teleport he is far more vulnerable to characters stalking an appropriate zone outside his DJ peak, and he also can't really challenge them if they're doing it right because his important hitboxes are in obvious and exploitable locations. This makes it extremely difficult for him to land with safety because he lacks the ability to effectively threaten anyone, and it's incredibly easy to setup little traps that cover all his recovery options as long as you remember to stay below or away from his Fair. His airdodge isn't particularly outstanding in any way, either.
You'll find a similar (though less ridiculous) comparison if you do it with Yoshi or Peach. The jabs and Bair traps that like 10 characters use to cover Ness's options during gimping or freefall? Yoshi can superman those and keep going until like 300%. On a side note, I'd argue Peach's umbrella kind of gives her a ghetto DJ sweetspot alternative anyway that's functionally the same, but that's neither here nor there. End of the day, Ness has none of the advantages they have. While they might be able to live without DJ sweetspot, Ness doesn't have massive invincibility abuse, or super armor abuse, or a repertoire of crazy tricks and boatloads of priority to fall back on to cover his faults like they do.
Ness blows.