Fetching is far more than simply someone
attractive - someone who can be called fetching has a certain
aura about them:
they wash over you.
You can feel them in your spine as they pass you by and smile their
jolly smile.
The word has kinship with cuteness - but less in the sense that the target person is cute, than the idea that they bring out the cuteness in you. You're forced to associate them with a word like fetching which is an infinitely cute way to describe someone - it almost makes you giddy and heady to say it.
You would only call someone fetching if there's a good chance of you swooning sometime in the near future.