For how to implement platform specific code (iOS and android ads run differently) read this: https://github.com/libgdx/libgdx/wiki/Interfacing-with-platform-specific-code
On how to implement admob on android read this: https://github.com/libgdx/libgdx/wiki/Admob-in-libgdx
admob or mopub ads on libgdx roboVM backend (iOS backend) use these: https://github.com/BlueRiverInteractive/robovm-ios-bindings
Thats all you need to know. And yes a lot of people have been in your position and managed to implement ads on both mobile platforms. Including myself.