Yeah, most of King K. Rool's issues can be traced to his slow mobility. Combined with his large frame and lack of low-startup lag attacks outside of neutral aerial (and in rare cases jab), opponents can just knock him around before he can get any hits in. His belly armor can help alleviate this, especially with his forward tilt, down smash, and dash attack, though it is limited and shouldn't be abused. Plus, if your armor is on the verge of cracking, you're pretty much forced to do away with those moves until it heals.
His current running speed is 1.585, compared Donkey Kong's 1.873 and Bowser's 1.971. I think boosting it up to around 1.7 or 1.8 would do wonders for his playstyle, allowing him to keep up with faster characters and punish them more easily. I'm sure I speak for everyone on the K. Rool boards when I say I hope the devs have been paying attention to how King K. Rool has fared so far and are making the necessary changes to make him better. Version 3.0.0 is right around the corner, so all we can do now is hope for the best.
Remember, King K. Rool's gameplan is to read the opponent, punish them hard, and keep them in the disadvantage stage. He's not the kind of character where you can just throw out projectiles and hitboxes willy nilly like his armor mechanic would have you believe. Each of your moves serves a purpose, after all one of his advantages is the sheer utility of his entire kit.