mainline now deletes the downloaded .deb files after installing them, so that cache/tmp doesn't just grow forever.
You may want to manually sudo rm -rf ~/.cache/mainline one last time to clear out all the old kernel packages. (Yes, you probably will need sudo to delete files from your own non-root home directory. This is a known bug I plan to fix sometime.)
If your /tmp is not tmpfs, then you may want to sudo rm -rf /tmp/mainline* too.
After that, cache will still grow, but it will only be the html index files listing info about the kernel packages, not the .deb files themselves. So the space used goes from ~1G to ~15M