The question asks, ‘Was Keats ever criticized for standardizing generic beauty?’ and the answer is ‘yes—indeed, he was criticized along these lines by his own fiancée, Fanny Brawne!’ The evidence for this appears in two letters from Keats to Brawne:
Why may I not speak of your Beauty, since without that I could never have lov’d you?—I cannot conceive any beginning of such love as I have for you but Beauty. There may be a sort of love for which, without the least sneer at it, I have the highest respect and can admire it in others: but it has not the richness, the bloom, the full form, the enchantment of love after my own heart. So let me speak of your Beauty, though to my own endangering; if you could be so cruel to me as to try elsewhere its Power.
John Keats (10 July 1819). Letter to Fanny Brawne. Letter II in Harry Buxton Forman, ed. (1878). Letters of John Keats to Fanny Brawne. London: Reeves and Turner.
When you pass’d my window home yesterday, I was fill’d with as much admiration as if I had then seen you for the first time. You uttered a half complaint once that I only lov’d your beauty. Have I nothing else then to love in you but that? Do not I see a heart naturally furnish’d with wings imprison itself with me?
John Keats (undated, but c. March 1820). Letter to Fanny Brawne. Letter XXIV in Forman.
Brawne’s letters to Keats were destroyed by him, but we can read between the lines of Keats’ replies to deduce the nature of her complaint.