Is App Delete in the app Store safe?

I don't know anything about that particular tool, but note that those kinds of app removal tools are imperfect at best, and there is always a danger that they will remove something they shouldn't or not remove something that they should. If an uninstaller is needed, the app in question should provide one. (Not all do... generally, if an installer is required and no uninstaller is provided, the app is likely to be junk that you should avoid.) If an uninstaller is not needed, just drop the app in the trash. You could stress yourself out about a few small settings files left behind, but since they are a drop in the bucket on modern hard drives and will not impact the machine's performance in the slightest, that's unnecessary.