default controller route for mobile

Not necessarily. While it is good to have a responsive site (which is styled differently for different devices/screen sizes), you can also have a separate view completely for the mobile site (if it's drastically different).

However, you should be doing this either by having a separate mobile subdomain or similar (m.example.com) or by detecting device on the server side (if you can reliably) and serving the appropriate view (but still using the exact same routes).

It really depends on your requirements.