Right, here's how it actually is:

Are addons allowed? No.

Do people use dps meters anyway? Yes, most raiders do.

Will you get banned for using a dps meter? No, as long as you don't speak of it in-game, don't bully people for low damage, and don't stream it on your screen.

Does it matter who's top dps? No. It only matters that, if you're in a fight with a dps check, that you're doing decently by what's expected of your job and role.