He's not an issue by how good he is, but by how cheap he is. It's just that those who think he;s an issue will complain like hell, while those who don't will spend their time countering him because the former is far larger and more vocal, it means that people will immediately rush to Diddy seeing the majority, seemingly everyone crying over his OPness, trying to use him for easier wins, or for those like me, to see his weaknesses by using him myself. I can hold my own against a Diddy up to slightly more skilled than me. He's beyond a small issue. the truly good can get around him, or will learn to.
I'm saying cheap because dthrow-uair-1 less stock-loss on FG is extremely annoying, and kinda fits the albeit volatile definition of cheap. people think he's cheap, think he's OP, rush to him, all the unprepared lose, and the cycle continues until eventually the pros start maining him, and all of a sudden he's top tier. This happens a lot in Smash, but never to this extent. It's usually a bottom/low tier getting recognition, or vice versa, a former top tier getting taken down or having the meta go all anti-*insert said character here. good example is Brawl ROB and G&W* on that character.
If it weren't for all the complainers Diddy might not even be considered high tier.