If you have an already active tmux session, and you connecting via Putty or mRemoteng. Do the following to have it use VT100 termainl to not have to redeploy putty.

1. Open putty
2. Window > Translation > Adjust how PuTTY handles line drawing characters -> Enable VT100 line drawing even in UTF-8 mode
3. Go back to session in Putty, save to default.
4. Now open a Putty session, or a Mremoteng session

Lines should be drawn properly.

Source: tmux in putty displays border as 'qqqqq' or 'xxxx'