I was looking to clean up my admin panel a bit. And was wondering is there a way to delete applications that aren't installed?

You have to delete the relevant folders from the applications directory on your server. Folders which are present but which don't represent installed apps show as uninstalled ones in the ACP.

If you do not self-host, I don't believe there is a way to delete those yourself. Possibly contacting Support they can do it for you but it seems like a pointless task. Just deal with it? Lol.