Plato has a notion of Beauty (Kalon) as a Form; so there it's Ideal - as all Forms are ideal; but given the close association of the two Forms - Beauty and the Good (Agathon), the word Beauty is not now perhaps the best translation of the word Kalon - where it's closer to noble, fine and excellent.
It's this notion that Keats refers to at the end of his Ode to a Grecian Urn:
Beauty is truth, truth is beauty - that is all
Ye know on earth, and all ye need to know
Beauty, in this sense is closer to the aesthetic notion of the sublime - mixing in the Great or greater than the Great - ie beyond and therefore Transcendent; but it has gone through many other difficult changes: for example, Baudelaire and Hugo mixed in the Grotesque; this signalled for the aesthetician Susan Sontag in taking the long view of modernity in Europe (and through Europe, elsewhere) that our era, like some other eras
are too complex, too deafened by contradictory historical and intellectual experiences, to hear the voice of sanity...the truths we respect are born of affliction. We measure truth in terms of the cost to the writer in suffering (review of Selected Essay by Simone Weil).
So also painters like Munch, Schiele or Bacon.
Notice the close connection Sontag implicitly assumes to Agathon, even for art born in affliction - it affirms it's authenticity.
But the proverb is not referring to any of this, but purely to subjectivity; it's comparable to another proverb:
One mans meat is another mans poison
Still, one can connect these views (Subjective to Ideal) by noting that though the valuation of Beauty differs, man by man, culture by culture; that there is this kind of judgement is a kind of constant ie an Ideal Form.
For example, take calligraphy; what is considered as beautiful script in medieval Europe, Arabia and China differ in particularities, in their form; but still, such a notion does hold; and one should expect it to hold - one has to step a little higher to notice ie more ideally (or abstractly).