Is it possible to find out who created a tag in Git?

You can use this command to show git tags with date, message and author email:

git for-each-ref --format '%(refname) %09 %(taggerdate) %(subject) %(taggeremail)' refs/tags  --sort=taggerdate

This is the result of this command when I launch it on Material-UI project:

If it's an annotated tag, then yes. You can git show it just like any other object. If it's a lightweight tag, then I don't believe so. Just one of the reasons why you should always annotate the tags you're going to share.