I suspect you have installed either Lutris or Steam through flatpak.

Try installing both using the same method or install MangoHud both as a flatpak and a system package.

Also, I recommend enabling MangoHud with 'MANGOHUD=1 %command%' instead.

It doesn't scale properly on my HiDPI laptop display, which I use at 200% scale (Plasma 6, Wayland). Some things are too big and others too small. Hopefully GIMP 3 will bring good scaling support with GTK 3. Still, I love GIMP and will continue to use it over any alternatives.