There is a little more to the series at this point than the 30 years old Link to the Past.
More seriously, Tunic also draws upon newer Zelda titles as well as games outside the series. In addition, many 2D Zelda games have come out since the time of the SNES, not even counting re-releases like the recent remake of Link's Awakening.
Tunic doesn't just evoke Zelda aesthetically as a substitute for developing its own aesthetic! It's a loving homage to a series that absolutely materially inspired the core gameplay, too.
I haven't had a chance to really play Tunic yet, but I just get the feeling that calling it a Zelda clone is unfairly reductive to both. If Tunic and Okami are both Zelda clones, but not clones of each other, something needs reevaulation.