I'd say this match-up is 55-45... 60-40 Peach.
The best Diddy I have played was Dekar's, and it was only one friendly set at Devastation. I wish I could tell you guys what to do, but honestly I can't describe the thoughts that go on in my head when I'm fighting sometimes...
If you are playing smart, Diddy will have a VERY hard time killing Peach. Seriously, float is awesome in this matchup, but you have to use it wisely. Diddy has a few tricks up his sleeves to knock us out of the air, so be careful of peanut gun, bananas, and other aerials of Diddy's, notably fair and bair.
Bananas are probably the most annoying thing to deal with, but honestly Peach can make quick use of them with her awesome glidetosses and item capabilities. Make Diddy pay!! Don't forget to auto cancel aerials to pick up bananas too... but dash attack works ok too I guess. Be super careful where you land from a float if you see Diddy has both his bananas out, the banana setups can be really nasty sometimes.
Our goal should be to juggle this f***er in the air as much as we can. Not only that, but we want him to have a fresh supply of our veggies in the match. If you feel you are being pressured too much, try to get away as fast as possible and reset the situation with a reverse glidetoss.
We can be a mean girl when Diddy is trying to recover. Use turnips to mess up his recovery and use your float to hog the edge. Lately, I've been trying to go off stage more for edgeguards, so if you do that, be careful of his side B, it's annoying lol.
Get comfortable fighting on neutrals against Diddy and try to get Battlefield if you can with the striking system... if not SV is alright too. FD should definitely be the first you strike/ban. If Pictochat is legal, it's up to you whether you'd rather fight on Picto or FD. Counterpicking shouldn't be a problem for Peach against Diddy. Brinstar can work, but be wary of bananas, they can bounce on the log thingies lol. Rainbow Cruise is an obvious choice... YI might work... Lylat could be annoying too. It's really wherever you're comfortable. Now that I think of it... Frigate might be a good choice too.