I just tried Overwolf's in-game FPS counter and it did not look good.
I think FPS counter should be something you'l see when you look for it but blends with the background when you're focused on playing. I'd make it like half the size, with a simple font and light gray color with like 80% opacity, like steam has it. All of this could also be customizable in the settings. I am the type of gamer that wants to be aware of their FPS so please give it an upgrade.