Yes, it's true for most add-ons, even though Stylish is quite different. It only apply some CSS instructions, this CSS code replaces the CSS that's styling Wincustomize in this case, not adding anything significant at all.
When you don't surf Wincustomize, this code doesn't apply at all so you won't feel any change in Firefox's performance.
Anyway, it doesn't matter, if you're happy from your setup - enjoy it.
Thanks for the feedback anyway.