How can I uncompress a tar.xz thats been compressed on ubuntu and uncompress it on mac with the archive utility?
Archive Utility doesn't currently support .tar.xz files. You can provide feedback to Apple at apple.com/feedback.

While Terminal can uncompress .tar.xz as you note, if you're looking for a GUI-based solution you can install:

 The Unarchiver

which has support for .tar.xz.