I've done some tricks to make WPP to act like I want; this may be useful for someone with my same problem.
So, the problem: the function get_mostpopular() returns a string, with
<ul><li><a> tags; I want to let have a class like "current-menu-item" in order to make it bolder (or what you want to do, that's doesn't matter).
You can customize the function like you want, taking help from the original get_popular_posts() (the one used by get_mostpopular() that returns the string).
Naturally this is MY semplification, it will be good if plugin's owners makes this changes as default; I think it's better to return an array to let you play with it as you want.
Hope to help someone.