Diddy isn't very "powerful" in terms of instant damage, but his method of stage control racks up damage at a decent rate. That said, the lack of power doesn't seem to "hurt" Diddy in matchups since he has the tools to make up for that. After that, he has a number of viable kill moves and setups into them. Iirc, glide toss -> dsmash kills some characters in the early 100 percentages (fresh)? If not, it knocks them back pretty far to setup for edgeguarding.
Tbh, I'm just trying to understand how we're using the term "power" here. If it's killing power, Diddy definitely has it. Fair and dsmash are solid finishers and I think even bair has its uses in edgeguarding. If it's accumulating damage, Diddy can't do it as instantly as someone like Ganon (or kill as early), but it goes without saying that he's more liable to get more hits in over a shorter period of time, and unlike Ganon or Ike, he has actual setups into those kill moves. One vital difference between Diddy and such characters is that Diddy has an easier time landing the moves that he needs to just from standard game play. Characters with slower, unsafe moves often have to mind game their way into their laggier moves.
