But what he became instead was far less interesting and imposing. Its true a villain doesn't have to just be an imposing dark figure, but if a villain is not going to menacing in that way, they need to have other qualities and features that make them interesting or imposing instead, be it intelligence, wit, foresight, psychological aptitude or whatever else.
Whether you're talking about Joker, Hannibal Lector, Darth Vader, Voldemort, Thanos, Anton Chighurh, Hans Landa, Le Chiffre or whatever well depicted villain, they all have qualities or traits that make them especially menacing or intriguing, or captivating to watch, but Kylo simply doesn't process that. Infact, towards the end of TLJ he almost becomes annoying and frustrating to watch instead. By that point I was bored and uninterested in his ineptitude and petulance.
See, what they should've done is lean INTO that. Lean into the fact that he's a shitty substitute for Vader and have him lash out. Not every villain NEEDS to be imposing or a controlling presence. Most evil people AREN'T that way. Not everyone needs to be Vader or Palpatine.