Are jackets worn my men for afternoon performances or is it more casual?

The overwhelming majority of men will be without jackets. Casual is fine.

You'll see everything imaginable on others at afternoon or evening performances. You may as well dress for whatever you're doing before and/or after.

No dress code whatsoever. BUT you will see elegantly dressed people & total slobs. IOW, whatever turns you on.

I'm embarrassed to say that I went to a matinee in shorts one time. Visiting NYC one summer and while we were touring we stopped at the TKTS window and they had discount tickets to a play that we really wanted to see. We didn't have time to go back to our hotel, so we went as is. I don't recommend it, but the point of the story is: casual.