This is a fascinating question to consider, as a careful reading will reveal that this poem is about Keats, acting as a lament for his life and as a pageant to his work and values, but also about the poet's own beliefs; the lament for Keats serves as something of a springboard that Shelley uses to reflect on his own attitude towards life and death. Particularly at the end of the poem, Shelley asks both himself and his reader "What Adonais is, why fear we to become?" Note how he presents his philosophy towards life and death in Stanza 52 of his poem:
Life, like a dome of many-coloured glass,
Stains the white radiance of Eternity,
Until Death tramples it to fragments. --Die,
If thou wouldst be with that which thou dost seek!
Shelley here argues that life in a powerful simile is like a "dome of many-coloured glass" in that it reflects imperfectly the glory and the "white radiance of Eternity." Death is actually something that is the antidote to the many cares and worries of life, reflected in that dome.
It must be noted however that the "Preface to Adonais" and early stanzas make it clear that in addition, Shelley is reprimanding the literary critics whom he calls "cankerworms" of "savage criticism" and whom he blames for a "rupture of a blood-vessel" in Keats' lungs. In conjunction with this important point, it is fair to say that this poem is about Keats but also about Shelley himself and his ideas and philosophy which are advanced in this poem, using his lament of Keats as the medium by which his thoughts can be shared.