I certainly don't think Diddy should be banned, but I disagree with many people in this thread who don't think Diddy is the best. I think Diddy is clearly the best character right now.
Just because Diddy doesn't place in the top 3 at every single tournament and that Diddy can be beat doesn't mean that Diddy isn't a step above the rest of the cast. No, Diddy is not broken. Yes, Diddy can be beat. But Diddy is the best character by a decent margin, in my opinion. No other character has the combination of Diddy's movement and kill power, and the downthrow to up-air is what puts him over the top.
It doesn't bother me in the slightest though because there is a 99.999% chance that Sakurai will nerf Diddy anyway. The question is when, not if. Sakurai stated in an interview recently that they just recently started working on Mewtwo. Realistically, Mewtwo will make Nintendo a lot of money, and even more SSB4 characters will be developed down the road. The point being, SSB4 is a game that will continue to be tinkered with for some time to come.
Sakurai is NOT ignorant of which characters are the most competitively viable. There was a very clear correlation between Japan's first tier list and which characters received nerfs.
So, with Diddy now as the only S tier character in Japan, nerfs are absolutely inevitable. And probably not just to only his downthrow like some people are thinking (although that's the most obvious nerf). There's a solid chance he will be nerfed across the board, similar to how Rosalina received literally about 20 different nerfs in the first balance patch. (Which people seem to forget, judging by all the people who want to use Rosalina as an example of how the meta can change. No, people didn't simply "figure Rosalina out." Rosalina was seriously nerfed. If she hadn't been touched she probably would have dominated Smash 4, so she needed nerfs, and she received them.)
So let's just appreciate this moment in time for what it is; It's the era of Diddy, which will soon come to its inevitable end. The only question is whether Diddy will be nerfed before or after Apex. My money is on after Apex, but the next patch is liable to hit us at any time. At the absolute latest, Diddy will be nerfed by the time Mewtwo arrives.