EDIT:
~/.tmux.conf
set -g status-interval 2 set -g status-right "#(vmstat -SM 1 2 | awk 'END {print \"cpu:\"100-$15\"%\" \" mem:\"512-$4-$5-$6\"M\"}')#[default] %R"
"512" replace your total memory, e.g. 1024, 2048. get by "free -m". total memory don't often change, so not in command.