Can I checkout github wikis like a git repository?

You can now!

git clone https://github.com/user/project.wiki.git

or if you use ssh

git clone [email protected]:username/project.wiki.git

You can't check out the wiki but you can set up a website that you can checkout and edit that gets regenerated on a push.

They call it GitHub Pages. Admittedly, not the same as a wiki though.