Misconceptions
King K. Rool is one of the most misunderstood characters in Smash Ultimate, primarily due to his reputation at the game's release. Because of this, learning the character can be quite difficult, as it's hard to find a reliable source of information outside of the well-maintained SmashWiki pages. Here we'll go over those misconceptions to the best of our ability.
"King K. Rool is a zoner."
Easily the most common of the bunch, King K. Rool is far from a zoner. K. Rool's projectiles are simply too committal for him to play a zoner playstyle in most matchups; trying this against a remotely good player will result in you getting hammered. He can be better-described as a tank, because of his armor and burst range often having him force his way in to deal massive damage. If you're the type of WiFi K. Rool that tries to do Crown -> Blunderbuss over and over again, you're not even going to reach max GSP, you've gotta shape up. If you want an example of a real zoner, look at this video.
"King K. Rool lacks noncommittal options."
K. Rool is a superheavyweight, which does mean he often has to commit, but this statement often comes with two clauses: ignoring his options entirely, and ignoring the nature of fighting games. K. Rool's aerials are ungodly safe in a myriad of situations, and it's often the fault of the player - not landing properly, not buffering an option, not abusing safety - if they get punished on shield or whiff. You have to commit in fighting games by their very nature, it's about how you commit and what comes next. It's very easy to say not to commit, but in practice you're often committing a lot without even knowing it. Improve your decision-making, review your replays, and you'll quickly see what I mean here. K. Rool is a character about getting that one crucial hit in and blowing the opponent up.
"K. Rool's UAir is terrible!!"
This one is somewhat reasonable, until you actually look into how the move is meant to be used. It can string and juggle like any other UAir, only you can't fast fall with it because of the hop. This is fine, it's just different. You use UAir primarily as a movement option, recovery tool, or an aerial smash attack. This move is crucial to K. Rool's success both offensively and defensively, and you need to accept that. These three guides will explain the move's benefits better than I ever could, though, so I suggest watching them in sequence to better understand what this move is actually for.
"K. Rool's recovery is really exploitable and sucks"
Just like any other recovery option, K. Rool's Propellerpack has holes in it that can let some characters really torture him offstage, such as Mii Gunner's Grenade Launch and Corrin's Dragon Lunge. However, it's also one of the greatest superheavy recoveries in the game on distance alone, being very difficult for some characters to contest. Players who bemoan K. Rool's recovery are often referring to the times where they try to recover diagonally after burning their jump; this is suboptimal and a big reason why you lose your stocks. UAir can allow K. Rool to drift down next to the stage, defending himself against ledgedrop aerials that his diagonal recovery is vulnerable to. If they attempt to punish UAir, K. Rool can fast fall like a brick and still make it back. The propeller can also be adjusted through swerving. His recovery is great, just not perfect, much like any other character's.
"K. Rool is combo food"
The buzzphrase to end all buzzphrases in the Smash Community, and often the first sign that you don't understand the nature of superheavies in Smash Ultimate. Every superheavy is "combo food", and so are every big body and heavyweight. Despite this, many find success, including King K. Rool, and that's because unlike many superheavies, he does have valid combo breakers, such as his NAir, which quite literally flips disadvantage on its head. The first thing you have to accept is that your stocks start at 50%...and then you're living for an upwards of 100% further, as if you were a normal character. Not only that, you have the best survivability of any character in the game, you can quite literally outlive some characters in this game and win solely off of that. In fighting games, you're going to get hit, and you're going to get combo'd, this is normal stuff. This is more a matter of acceptance and moving on than anything: what truly matters is actually killing the fat bastard.
"K. Rool is slow"
Quite frankly, I don't know where this comes from. K. Rool's frame data is great for a superheavy, and his range and size make his low numerical speed amazing. You can look at the numbers on UFD, but then when you gauge K. Rool's speed with other characters in practice, you soon realize that his actual size is what balances this out, making him a deceptively quick character. Not only that, K. Rool's burst range is gigantic - think Dash Attack, Pivot Cancelled FTilt, and Boost Grab - which lets him get in on his opponents from around a quarter of the stage away. He has so much going for him in this department, even his mediocre air speed - which is a legitimate flaw - is largely offset by his massive FAir and decent run speed to assist it.