If you're making one out of an existing synth, it just depends on the source's terms of use. If it's a part of MacOS by default there's probably a section for it in the user license agreement. A quick search on my end pulled up some posts saying that they're okay to use for non-commercial purposes (which would include use in UTAU so long as you're not profiting from it), but you'll probably want to verify that before going through with it rather than just taking random internet users' word for it.
This said, I don't know your experience level in UTAU, but you should know that working with artificial voice sources for voicebank development can be a little more difficult than working with natural singing data just because you have to play around the quirks of it (though its not as difficult as jinriki since you still have control over the TTS).