Diddy is still being explored, but I think overall he's still pretty mid-tier. He doesn't have many MUs where he wins by an overwhelming margin; I don't think he has many significantly bad ones, but most of them tend to be negative imo. He's sort of like Squirtle in being sort of gimmicky.
Ness I definitely believe is better than Squirtle in the long run, but not by a huge margin. I think match up wise they're similar, though I think Ness is slightly more balanced in his MU spread. Its also important to note that Ness has been explored much more than Squirtle.
Ganondorf is slowly dropping down the tier list. While he's decent at playing defensively versus rush down characters like Fox and Mario, he doesn't fare very well against other defensive characters like Ivysaur, Zelda, and Link, especially because he has no good answer versus projectile users. His recovery his pretty awful and almost the whole cast can edgeguard it with ease. His attacks are powerful but slow, so Ganon has to rely a lot on reading the opponent, which is much more difficult for Ganon in PM due to the many new changes being brought, including the ones that he can't deal with well. Many people know Ganon's game compared to the newer characters like Squirtle, so I don't think I'm being too farfetched when I say that Squirtle is slightly more balanced than Ganon.