I will state this once; Diddy Kong is not that overpowered. He and Sonic have great attributes (except Sonic's is annoying and some are countered by other characters), but you have to remember that Diddy Kong is light, and his recovery is a charge type recovery that is a bad idea to use under a stage.
Sure, everyone says his U-air is overpowered, but people will adapt to it and find ways to counter it. His throws might be annoying and link into each other, but people can spot dodge or roll away from it. Monkey Flip may be good, but a spot dodge or rolling the opposite direction can counter that. F-Smash has great knockback power, but that's the main ground attack Diddy Kong players use; exploit it.
Diddy Kong, to me, is not very threatening or scary. There will be ways to develop strategies against Diddy, and characters will soft counter him to a lower position. The Links can fare well against Diddy Kong, and Ganondorf (in my experience) can stand his own.