Diddy approaches by using bananas to trap the opponent as he moves in and reacts to their defense. Blocking a banana pluck may get you grabbed, for example. When two attacks are layered on top of one another like this so that defending against the first will get you hit by the second, its called a frame trap. Diddy might rely on frame traps more than any other character in neutral, besides maybe...Ness? Copy and paste from another thread because I'm lazy:
Since the damage and KB nerfs Diddy needs to get in cleanly many more times than before and the banana is how he does this. Opponents need to be able to stop Diddy from setting up by either patiently plucking it behind him and immediately jumping, z-catching, and fast falling back to the ground, or aggressively b-reverse plucking the banana towards the opponent from mid range and approaching with fair, dash grab, nair, and empty hop mixups as the banana falls and traps. If you can stop Diddy from setting this up successfully enough he will not get the damage output he now needs.
This is a lot harder to deal with than it sounds on paper. Even if you defend against the banana really well, say, by z-catching it and immediately throwing it away or back at Diddy, or quickly jumping and destroying it, you're still being forced into a situation where you have to do something and the grab/fair mixup is on the way. It's like how Dedede mains use gordo, it can be defended against but you have to do something, which allows Dedede to come at you the way he wants.
Except Dedede's are explaining why Gordo isn't trash. Diddy's use this strategy to make bananas the best projectile in the game. There are dozens of of ways to approach with bananas in Smash 4 because now we can repluck a banana out of our hand after the initial spawn pluck, in addition to four directions to throw and a z-drop. Different setups require different defense maneuvers. Because the Diddy player will know which way he will approach out of many setups and the opponent has to guess or react, skilled banana approaches are the most free mix up in the game. Opponents will want to stop Diddy from setting up in the first place to stop these mixups before they happen.