I have always loved this poem, with its childlike tone and Biblical language ("thee" and "dost thou"). It is truly a Song of Innocence, lyrical and beautiful.
I do dissent mildly on the question of whether it is a pastoral poem. It does not fit the strict definition of pastoral literature, but it certainly alludes to elements of the pastoral. The discussion of the lamb's physical qualities and the description of how Christ cares for the lamb are pastoral in nature.
This poem evokes feelings of tenderness because of its innocence and holiness. What a wonderfully simple poem with the first stanza concentrating on the lamb itself and the second stanza focusing the lamb as a symbol of Christ: a piece of literature truly belonging in Blake's Songs of Innocence.
As to the question of it fitting the definition of a pastoral poem, I'm afraid my answer has to be no. A pastoral poem is defined as a composition focusing on the life of rural shepherds, often idealizing them. Although Christ is a shepherd, He is a metaphorical shepherd. We are his sheep. Perhaps if the poem delved more into Christ's life as shepherd, focusing specifically on how His life in this regard is close to perfection (even on earth), I could consider it. Christ is both the shepherd of sheep as well as the lamb sent to the slaughter. However, the poem focuses more on how Christ is the lamb itself (almost the opposite of the shepherd metaphor). Thus, the poem cannot be called pastoral.