Would it make sense to use SFML if you just want to create a GUI application?
No, not really. If all you want is a GUI there is much better libraries out there that are intended to be used as GUI toolkits. SFML is intended for 2d rendering, so if you need that you are probably better off embedding SFML inside one of those other GUI toolkits.
Or is SFML too resource heavy?
SFML is as heavy (or light) as you make it out to be, if you tell SFML to load 100 textures then it will.
I thought about decreasing the frame rate limit but maybe SFML is not even that resource heavy..
Frame rate doesn't really affect how "resource heavy" SFML is. Sure a higher FPS will use a bit more CPU, but I doubt you would notice a difference between 60 FPS and 30 FPS on the CPU usage.
Imagine you run an SFML application on over 20 clients(server is intended for 30~ clients)
Are you talking running 20 instances of an SFML application on a single machine? Or are you talking networking here meaning a server handling 20 simultaneous network connections?
Bad idea.
Please post something more relevant to the discussion instead of just two words that don't mean that much.