Xamarin : Should you use an MVVM library and which is the best

With WPF I use "MVVM Light Toolkit from galasoft" but with Xamarin does it bring more functionality or simplicity to use a MVVM framework ?

This part of the question is OK and I'll answer it, the rest is not and it is strictly prohibited here to ask which library is the best because there is no such thing as the best library.

Speaking of the above there is a major difference between WPF and Xamarin. You can't do some major things in WPF without passing some variables from the backend to the model which deifies the purpose of MVVM, and that's where MVVM libraries got in place, to resolve that. As this is unnecessary in Xamarin, they bring almost no value and a nice pile of problems.