Well, C and C++ won't do you any harm in mobile programming. You can use both with Cocoa Touch or the Android NDK, and sometimes you need to for performance. Also, sometimes you want to because you want to write code that runs on several platforms. In that case, assuming you're not just using a third-party framework, you can write your app in C/C++, and compile in platform-specific wrappers for required native elements (for example, the only way to get an OpenGL context on iPhone is by using the native CAEAGLLayer class).

So it's like the more tools you have in the drawer, the more options you have for getting the job done. You'll find you have a preference for certain ones as you specialize in various kinds of tasks. If you only do UI stuff, C might not be your all-time favorite. I would worry more about the kinds of things I wanted to get done, and then learn any tool I needed to do them.