Where wert thou, mighty Mother, when he lay, When thy Son lay, pierc'd by the shaft which flies In darkness? where was lorn Urania When Adonais died? With veiled eyes, 'Mid listening Echoes, in her Paradise She sate, while one, with soft enamour'd breath, Rekindled all the fading melodies, With which, like flowers that mock the corse beneath, He had adorn'd and hid the coming bulk of Death.
Mythology note: Urania (also known as Aphrodite) was Adonais' mother (and also the goddess of love). The speaker wonders why she wasn't there when her son was killed by a boar (which is how Adonais died in the myth).
The speaker figures that Urania was lying around in paradise ("sate" means sat), listening to a lover (with "soft enamour'd breath") speak sweet things and generally just chilling out without a care in the world.
This scene of pleasure was so great that she didn't notice Adonais' death. The speaker, using a simile, says that the pleasurable things act like flowers that cover up ("mock") the scent of a dead body ("corse").
Now, some myths say that the boar that killed Adonais was sent by the jealous god Artemis, who wanted the beautiful youth gone. Is the speaker comparing the jealousy of Artemis to the jealousy of the reviewers? If he really thinks that Keats died because of some bad reviews, then we'd say it's definitely possible.
Whatever the case, it seems that the speaker thinks someone should have been there to save Keats, just as Urania should have been there to save Adonais.