honestly I do believe this MU is even; I only see it in his favor due to Diddy's being better at naner control but thats a player thing.
The biggest thing for me in this MU is understanding how to implement item control alongside spin dash. you HAVE to understand how to do this to be efficient in the MU imo. You can pick up any item you roll over and naners are no exception. You can z-drop naners as u are doing any aerial spin. our DA and fair (by far the more important one) are good at picking up naners as well. PLEASE learn to dash cancel z-pickup ( iono the exact name for it). it allows us to use our amazing run speed to pick up naners without lagging due to some aerial or dash attack.
Also when fighting Diddy understand that diddy glide tossing toward you puts you at frame advantage. You can aerial OoS, but the preferred tactic SHOULD be shield grab imo unless u want to setup a string like sh fair to catch naner>glidetoss forward> stuff.
This is HUGE in the MU because u need to understand that staying in shield vs a diddy holding naners isnt all that bad coupled with up b if need be.
Diddy also has a fairly slow dair which equates to some good juggle potential for us. Speaking of aerial Diddy, always remember side b by Diddy can always be punished with a retreating bair ON REACTION. Due to the input of the move you can catch some diddys in some bad DI. Even if your reaction is bad and you miss the read you can just walk away, dj if they have a naner or whatever. Also once you have a naner dont just throw them blindly, use them with a purpose. You notice the Diddy likes to retrieve his naners ASAP, drop them and use them as bait. Diddy like to throw them back at you? throw them then run and sh fair or uair them while catching the naner automatically. Our speed already allows us to bait the crap out of the entire cast. With naners, we are able to actively pressure while being completely freed up in terms of movement.
When it comes to tosses we have Diddy pretty much beat. We can SDSC Glide toss, regular glide toss, SDSC jump cancel toss and regular jump cancel toss; the latter two being relatively the same distance as Diddys glide toss. even as we glide, we can alter spacing with retreating and advancing SDSC. We can punishing their own knowledge of glide toss punishing with a jump cancel toss to a fsmash punishing a spotdodge, shield grab, and aerial. Be creative here we seriously destroy this field.
When it comes to punishing his recovery, we have bair to intercept side b, spring to punish up b. Toss a naner into the mix and it becomes very difficult for Diddy, sometimes almost solidifying a gimp ( Glide tossing up a naner at the edge allows u to be aggressive offstage making any delay in Diddy recovery equal gimp by naner). Even if he grabs the edge Diddy has a hard time dealing with bair traps. or just simply standing outside his ledge attack range. If he tries to drop down and pull a naner, a simple run off spring could seal the deal.
Lastly I want to add something that is flat out hilarious imo. Diddy often is feared with a naner in his hand becoming one of the greatest punishers in the game. However, when a Diddy has a naner he actually has a huge weakness that really only gets abused by us; inability to deal with ASC pressure. Once he has a naner in his hand, we can attack in this manner with little repercussions. The one thing he can do is jump up and throw a naner; which can be caught by any aerial out of our ASC. Why is this so good? Becuz Diddy as a character without naner is not too bad, however Diddy with naners is the real threat. Versus Us? We have tools to make him have to pay for holding a naner in his hand for too long. We also dont have to deal with the real estate taken by grounded naners. We can even sit in spin dash and simply go right through naners as shown here:
http://www.youtube.com/watch?v=_DzX-FYONrw#t=4m54s
*Note: I dont play the MU nearly as well as I should in this game.
I do have some matches vs Vato where I play it somewhat decent. Ill try to put those up when i can. My wii's reader recently died : (
Lastly, dont get smashed by dash attack. you can grab it every time.