I use the mouse all the time in tmux and nvim, especially for resizing splits, scrolling terminal output, selecting text from tmux, and navigating my nvim file browser. tmux actually has a right click menu that is nice to use occasionally.
I got stuck on a job in Dubai (Government client - they also had us work in trailers on-site - no remote access allowed) with a Remote Desktop jump host where the mouse didn't have any pass-through to my terminal session - which mean I had to work for 8 hours a day without any ability to select/copy/paste with a mouse. I had tmux-installed on the remote hosts and for the next three months, select/cut/copy/paste/resize panes/etc... were done without a mouse. Seven years later, I've never once used a mouse with tmux since then. https://github.com/schasse/tmux-jump is the equivalent plugin that lets you jump anywhere you want on the screen.