Luke Skywalker was orphaned. PTSD. Survivor syndrome. Imposter syndrome. Finds out his dad is the galaxy's top fascist villain. Compassionate and big-hearted. Also fear and anger, impatience, rashness. TLJ didn't ruin Luke. It lifted the hood and poked at the actual person

Luke isn't the same character he was in the OT because nobody stays the same. The young golden hero of OT has fears and foibles and vulnerabilities. He's a person. I never got Luke more than I did after I saw TLJ. He was trying to save people. He was scared. He made mistakes.

Rather than messing things up further, he ran away, he hid. He thought he was a failure as a teacher. He didn't set out to murder anyone. Imagine the pain of him believing he'd failed Han and Leia's kid, of all people.

So rather than seeing this as a ruination of the character, TLJ added layers and revealed and remember also that he came around in the end. The whole point was you can hide for a time but sooner or later you have to stop running. Luke stepped up, he went out a hero.

Luke also fully came into his own as one of the greatest Jedi. It's like he had some final steps in his own training to complete.