Let's talk about Brawl Metaknight for a moment.
Brawl Metaknight straight-up invalidated a lot of the cast. Not only that, but he did it trivially.
In theory, winning the Game & Watch matchup was as simple as correctly spacing ftilt and upB, which in tandem beat every single option G&W had, save for phenomenally unsafe **** like coming in with Dair (which was also easy to beat on reaction for free).
Beating DK or Bowser? Tornado on landings; they can't do anything about it because nado beats every one of their aerials from the side, and their dair and nair are too big not to clash (and therefore lose) with nado.
Wario? Gets walled by fair really hard, and his air movement barely matters when Tornado wrecks air options so hard. He hits hard enough that capitalizing on mistakes can turn the match, but it's still a matter of "a handful of MK's options make almost every option irrelevant".
Luigi? Like Wario, except that Wario actually has air speed.
ROB? Big target, crappy dair, eats a lot of tornados (also, MK beats every one of his options on the ground pretty trivially)
Peach? Ftilt and upB and there's just nothing she can do.
You seeing a pattern? Fair, ftilt, upB, and nado together provide a toolset that just completely shuts down a lot of characters. His options are not just good, but way better than anything else. It would be like putting, I dunno, Vergil from UMvC3 (and drop the teleports from his moveset) in Street Fighter - ostensibly, the character isn't way out of place, but his options are just so far superior to the comparable options that it invalidates them - there isn't an anti-air in the game that can contend with a well-spaced j.2H (Helm Breaker), for example, and st.H is just so ridiculously huge and disjointed that it's damn near impossible to out-poke him. MK is that - a top tier who is a top tier because his options are obscenely strong. They could do virtually no damage and the character would still be obscenely polarizing.
Diddy though?
Diddy is not.
Diddy has a bunch of strong, potent tools, including phenomenal kill moves, but nothing that even borders on the silliness of Metaknight's ftilt or nado. Diddy is not overpowered because his options invalidate the cast. He's just safer than he needs to be and does more damage than is reasonable.