I kind of see what you mean, but tmux and mosh are completely different things and have (almost?) completely different use cases. Mosh is replacement for ssh.

Without talking about the other huge benefits of tmux, mosh only helps (and was designed to) prevent disconnections and other inconveniences when your internet connection falters. If for example your terminal unexpectedly closes you can't reconnect to your ongoing mosh session (this is an intended feature for security reasons).

Tmux on the other hand wouldn't help against a poor internet connection, but at least if your terminal or your ssh session closes you can just restart another one and wouldn't lose anything.

In other words, always use tmux, and tmux+mosh if your internet connection is poor.

mosh only supports reattaching from the originally started client - no attaching from different machines, no resuming sessions after reboots, etc. Also no splits, and no scrollback.

It's also extremely slow. Fullscreen terminals on my 30" display are borderline unusable with mosh.