Time machine not deleting Mobile Backups after backing up on external harddrive
Yes. It is safe to delete these backups after Time Machine has finished backing up the external hard drive. You can also disable local backups completely by executing the Terminal command sudo tmutil disablelocal.

I have done this on my laptop because all the content I use is stored in the cloud (Google Drive &amp; DropBox) and on remote private servers independently backed up.