Okay then, let's elaborate.
As previously stated: it's Diddy's place, manner, and usage that make me vote against him.
When Rare was given the DK license by Nintendo to make a new game for the SNES, they only had one character: Donkey Kong. From other Rare projects (Banjo, Ghoulies, Gemini, Kameo, even Perfect Dark), we can see Rare's philosophy on characters: there has to be alot of them, and no archetype should be left out. At the very least, a diverse cast is a must when Rare creates a platformer.
So Diddy Kong was born! Meant as a chirpy counterpart to DK's tough, stoic existence, he was an instant hit. And really, if none of the other 'Countries' had been made, I'd have no beef with Diddy whatsoever.
My problem started when we were introduced to such horrendously generic sterotypes as Candy(the next door), Dixie (the sassy kid sister), Tiny (the ironically-named, lovable thug), and Funky (the "cool" one).
These tagalong kongs supplanted the star of the games in later DKCs, and eventually ruined the otherwise mediocre collect-a-thon that is Donkey Kong 64. (On a personal note, I had sworn off of DK games until King of Swing and Jungle Beat helped me back into the fold.)
By this point, Diddy was no longer the "spritely sidekick", but the "crazy kid" of the series. This is where my argument gets a little hazy, as the distinction between the two is rather hard to spot. The best way to put it is how Diddy was presented in DK64. Corkguns? Jetpacks? Why? Diddy never needed these tools before, so why?
Because by this point, Rare had lost sight of what they were paid to do: make good games. They now had a "successful" formula, established with Banjo-Kazooie, which they would apply to all future 3D projects. If you think about it; once they nailed the DKC formula, not a thing was changed. This, as anyone will tell you, will hurt a series after only so many games. I'll give you three examples, in case the meaning doesn't sink in:
1. Sonic the Hedgehog
After Sonic Adventure, the 3D Sonic's fell apart. Too many bland sidekicks, stubbornly 2000-esque controls, and no changes to the overall formula have really damaged the franchise's reputation. The recent Secret Rings shows just how much for the better a formula switch can change a series.
2. Street Fighter
This hunk of arcade goodness refuses to change. How many "Street FIhgter 3's" have we sat through, wondering wjat was actually changed? Again, no adjustments to a proven formula really blemishes an otherwise good game.
3. Banjo-Kazooie
Only one game was fun. I'll give you one guess as to which one I mean. "Tooie's" reception as a re-hash is the most extreme example of how a tired concept (how many notes/puzzle pieces/honey combs can you collect!?) will utterly a franchise.
...Still with me?
So where was I? Ah yes, Diddy in Brawl. Since DK64's release, all Diddy's done is race in cars. He hasn't made any platforming appearances. This is because noone wants to touch a property with such uninspired characters. There is a reason Rare was sold...
To wrap things up (my library's about to close!), Diddy's image as a "crazy kid" doesn't mix with the rest of Brawl. Not the best conclusion, but I'll answer any extranneous concerns later from my Wii.