yes, mosh isn't required, except with mosh you get magic roaming, disconnection support, etc. mosh is the best way to do remote shells from a mobile device (laptop, phone, etc).
Isn't the 'disconnection support' (and by extension, roaming) one of the big points of tmux? This is one of the reasons why I never bothered with mosh -- I have screen/tmux, what's the point?