What are some successful musical performers out there, that are actually terrible, and would actually not have a chance at winning even any of the many 'X country got talent' competitions?

Though I disagree, Wierd Al Yankovic might be who you are looking for.

Weird Al is a treasure, plays multiple instruments and writes satirical lyrics.

Several bands that he mimicked commented that he captured their sound better than they did. Weird Al is a talented guy.

All this bubblegum autotune shit pop gets my vote. Some of those “artists” not only can’t play any instruments but they can’t really sing either. Not that I really care about vocals anyway. I like instrumental music.

The Rolling Stones.