I agree. Even if Samus has some tricks, the total advantage goes to Diddy Kong. I personally think this is because Diddy Kong is found to be Top Tier, while Samus a Low Tier. A matchup against Diddy Kong is hard enough to keep up even with my other mains like Zero Suit Samus or Marth. But when trying to go up against one with Samus, it's extremely hard since we have to keep up and counter the great qualities a Top Tier has, avoiding every defect that makes Samus a Low Tier. Although I have been proven wrong in multiple matches,yet I think a character can only compete against a rival three ranks higher at the most. If Samus is a D, I think she has a change against C, B and A ranks if mastered. Diddy Kong is an S. From there, I think it depends on the gaming level between the two players to overcome such difference between D and S. If the player is just as good as you or almost as good, it won't matter, since Diddy Kong holds the advantage to keep up with his/her mistakes. Ps. I know there's no Tier List yet, this is just an example.
The tips I gave are things that might help out, but I doubt they'll give anybody a victory against Diddy Kong. Truth be told, to defeat one, you'll need to play with all you got. Playing without compromising moves. Avoiding the use of moves with lag, great spacing, spamming if possible and only using grabs, Utilt, UpB, Fsmash if you're about a 75% sure it'll land the hit. This is definitely one of Samus' hardest matchups in the game.