Is C a good language for webapps compared to python and ruby?

It's a bit unusual to program a backend in C, but it can be done. Python and Ruby have a better community and better support for that kind of thing though.

If your programming in C, all you'll have access to is the bare bones. Also, you need to be more carefull about security (for things like buffer overflows). You'll have a harder time finding programmers for it (since most web developers might not know C).

The only reason you might want to write a portion in C is performance. If that's the case, then you'll only need to write a little bit of it in C, and even then I would probably use a more suitable language to call the C code instead of using CGI directly.