That's a problem of the first generation Prius then. Even a Nissan Leaf has absolutely no problem climbing mountains (they will murder the battery charge though – ask me how do I know – although you'll get some of it back as you go back down). A Chevy Volt would do it even better, and that's an actual hybrid. Electric engines have outstanding power relative to their size. The Leaf actual engine is the size of a football.

The Prius has been redesigned three times since then, so I'm not sure what that tells you.

Gasoline engines lose power at altitude, as it happens.