As someone who's in Elite Smash with
among other characters, I believe he's really good. I don't think he'll be topping any tier lists, but he's certainly fun to use. He may have been made to be a noob crusher in mind (which he really is a noob crusher), but he was also made to do things heavyweights normally don't do. Hence why he was given a counter, a reflector, and a long-distance recovery.
Because of this though, he's different from the other super heavyweights. You can't just rush in and use your weight to tank the damage. You need to play smart with him, trick the opponent, dare I say be "cruel" with your moves. Yes, his belly armor helps him win exchanges, but it shouldn't be relied on too much since it can break and continued use of his armored moves can slow the recovery process. Of all the heavyweights, he's the one you need to change up your approach most with. Crownerang and Blunderbuss deal decent damage, but what you should really use them for is baiting reactions and punishing accordingly. Both of these can encourage jumping and shielding, which you can easily counter. These can also encourage reflectors if the opponent has one and... so what? You have a reflector too. Dashing into up tilt is a good anti-air option, and forward tilt deals good damage and can tank a weak attack. I also find myself mixing up my moves with down tilt, as it comes unexpectedly unlike down throw so buried opponents have less time to react when I throw a forward or down smash. Up throw deals a whopping 20% damage, so I sometimes use that early on to get a lead or close the damage gap. If they don't react quick enough, I can get an up aerial as well... or read their dodge and punish.
K. Rool's weakest point is his terrible disadvantage state. Once he's being juggled in the air, his only option is to spam neutral aerial to armor through attacks, which is predictable and punishable. Propellerpack is a good recovery for a heavyweight, but once he's on the ledge it can be difficult to get back on against an aggressive opponent. Sometimes you can shark through the ground with the propellers, but after awhile they'll catch onto that and simply stay farther away. And considering he can't cancel or act out of Propellerpack, he's pretty much forced to recover low, which can be predictable to opponents capable of edgeguarding. If they land a back aerial you're one tech away from losing the stock, and some characters can even meteor smash K. Rool through the propellers. Like other heavyweights, he's also a pretty big target, which combined with his heavy weight, makes it difficult for him to escape combos, especially against quicker opponents. Because of all of this, your main game plan with K. Rool is gonna be to overpower them in the neutral and outlast them.
Like
, I've found that it's best to just have fun with K. Rool. That's not to say he can't be played seriously, but considering all the wacky options he has and all of his hilarious animations... why not have some fun along the way? That's what I've been doing since release, and though I am in Elite Smash I don't really care. Having fun is what's most important.