Currently, you could make the menu play well on mobile devices, but it requires a lot of extra effort. For 2014 Q1 release, we'll make the menu play even better with mobile devices.
Currently, the only easy solution is to hide the sub levels completely upon entering a certain viewport size. Take a look at the attached file for more details.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed