Zones of combat:
On the ground: Diddy is combo king here. Smart opponents can fight against his ground game, but many characters will prefer to take to the air against him, due to his great ground stage control with bananas. He is also good against aerial characters thanks to the way his peanuts arc, and his fast and ok ranged dtilt. Above Average
In the air: Diddy is okay here. His moves aren't really fast or really ranged, but each has a good use. Bair is fast and spammable with decent knockback. Fair has a pretty good range and good knockback. Dair is a strong spike. Uair has little range but can kill off the top. Nair isn't too hot. I'm not sure what Diddy's aerial mobility is, but he also has his Diddy Hump to move left or right while in the air. Average
Offstage: Diddy can recover pretty far, so offstage is a decent combat zone for him. His up b spikes, in addition to becoming a possible threat if the opponent tries to hit him out of it (rogue barrels do 18% with high knockback). He isn't as safe as some characters due to a lack of range on his attacks and speed, but he has projectiles to protect himself with and decent options in general (he can also wall cling / wall jump as part of his recovery, which is helpful in getting back onstage or edgeguarding). Average
Spacing:
Extreme Long Distance: this is bearable for Diddy. He has no projectiles that reach really far, unless you charge up a peanut all the way, and you can't spam this. I'll also assume a glide tossed banana does not reach an opponent at extreme long distance.
Long Distance: This is comfortable for Diddy. Some characters might be able to spam projectiles at him, but he has his own and he doesn't have a huge issue with closing the gap a little, especially with glide tossing which covers his movement with a moving hitbox.
Medium Distance: Diddy is comfortable here. Bananas can be thrown (backwards glide toss if you're being defensive, forward for offensive), peanuts can be used as an anti-air tool. This is his best range to begin combos at.
Short Distance: this is bearable for Diddy. He has some fast tilts (utilt, dtilt), but they aren't the most ranged or the fastest. He has a decent grab game, but he can't really start combos at this range.
Killing:
Killing Power: Diddy's Fsmash and Dsmash are his strongest moves. Many of the heavier characters can survive until 130-140% before a fresh one of these kills them. Fair, fthrow and ftilt are also decent finishers. He isn't too great at killing, so I'd say below average.
Gimping potential: Diddy has three spikes, a fairly long recovery, 2 projectiles (3 if you count 2 bananas & 1 peanut). He is pretty good at harrassing recovery. I'd say above average.