Last tested: May 10, 2019
If your instance is hosted by Looker, the .tmp directory is cleared on startup.
If you host your own instance, and this directory gets too large, you can safely delete certain files in .tmp when the instance is shutdown.
Admins may want to add something like this to their startup scripts:
rm -rf ~/looker/.tmp/jruby* ~/looker/.tmp/looker-tmp*
This will clear the .tmp folder of unneeded files on startup.