Yeah, some CSS and GTK+ interactions is definitely useful data to make conclusions about whether or not the Steam client or Source engine run well in Linux... (Neither of which (are likely to) use GTK+ anyway at that).
Clearly a polished GUI is not requisite for good compatibility/performance, but it certainly is a canary-in-the-coal-mine indicator for other less visible bits. Basically, if they haven't had time yet to fix things like unreadable menus, that gives you an idea of how close to RTM quality the underpinnings are.
To be fair I imagine the client itself is much higher quality.
This just looks like a php form someone threw together in half an hour. Valve has a "work on what you want" policy so I imagine most of their developers are more interested in building the games and steam itself than survey websites.
