My buddy has been trying to find a decent new main for him, guess he got tired of Fox. He's been using Diddy.
There are a few things you need to look out for, but if you play smart and see what is coming, Diddy is actually quite an easy win for Ike.
Of course, number 1 is Naner Control. Those Naners are the biggest obstacle that Ike will have to over come. Spot Dodging and taking to the air is obviously the best way to avoid, but you also need to be able to pick them up. Whoever has Naner control will most definitely take control of the Match. Personally, once I take a stock from Diddy, I toss the Naners and get them out of the way.
Another thing is his Dash attack, which has virtually no lag afterwards which set you up for a number of combos. A more common one in my opinion is his up tilt right after the dash attack, because at low percentage it continues to combo. One thing my buddy does is he will shoot a peanut to distract you and then come right to you as you try to avoid the peanut.
A thing to look out for is his Down Smash. You don't want to get too close at a high percentage because that is a really good kill move other than his forward throw. When I'm at a high percentage, I try to avoid getting TOO close to Diddy just like I would avoid getting Dolphin Slashed out of shield by Marth.
Of course your edge guard game has to be good. Timing your edge grabs is your best friend and when you are in the air over the side of the stage, you'll definitely want to be careful for his Down Air/Up-B because both of those spike big time.
I personally am trying to get into more of a habit of Nair-AAA combo, but Fair is probably your best friend against Diddy. Keeping your distance and shielding/dodging always good.
I almost forgot! His tilts are really good and definitely annoying. When over the edge, his down tilt "clap" can get annoying, nothing painful but just irritating. His side and up tilts are always good at knocking you back a bit.
And of course, his Fsmash is always easy to kill you when you are against the ledge. The first will knock you off the side causing you to drop your shield and the second fist (which seems to have a lengthened hitbox) will in most cases put you past the Blast Line.
I didn't want to make this HUGE post any longer than necessary, if theres anything I missed call me out on it and I'll tell you my thoughts.
To sum this all up, a lot of Diddy's attacks come from the core, which is his Dash Attack. Naners and Peanuts help set it up since the last person to throw a Naner doesn't trip on them and be careful with those smash attacks. Fair is my best friend when facing Diddy. Even if it does diminish, it'll eventually rack up some serious damage and has great knock back. Get him with this over the edge and you'll more likely than not knock him out.