Don't mind me, I'm just passing through.
On the topic, Diddy is far from one of the strongest doubles choices out there, but that doesn't mean he's unusable. It's just that, aside from banana peels, there is absolutely nothing about Diddy's move set that is exploitable or abusable in teams battle. Diddy is a beast in singles because of his defensive and offensive versatility; both of these narrowed a great deal in teams, however, because the sum of Diddy's play-style is greater than its individual parts. One of the most dominant tactics in teams play is that each player on the team may have a specific task (for instance, one may projectile-camp/stack on damage while the other one dispatches with KO moves). Diddy does not have a dominant move or tactic that rises above the rest as "unpunishable" or "broken" in teams, and even though bananas come close, having team attack standardized kind of thwarts how broke it might be.
That said, double Diddys is hilariously effective. Instant throw banana strings FTW.