I shot an HD 60fps video on my iPhone 12 Pro and uploaded it to youtube and it looks terrible!

I don't think you can. As far as I can see, youtube can't accept Dolby Vision. You'd need to edit in an Dolby vidision compatible editor, and output in a format suitable for youtube.

https://support.google.com/youtube/answer/7126552?hl=en-GB

(I'm far from an expert here, so someone might come along and correct me)

That is a good question for Google.

For real. You know, for a company that makes everything easy, nothing has been harder for me than trying to get iPhone 12 videos to look good on YouTube