The Waste Land is an epic poem. Broken into five main parts with 434 lines, The Waste Land is one seriously long poem. Epic poems are generally lengthy narrative poems, and Eliot's poem could certainly be classified as such, even though the poem itself does not follow any sort of defined story line.
Although Eliot's poem does deal with themes of death, dying, and destruction, The Waste Land itself could not ever really be considered an epitaph simply for the reason of its length; an epitaph is a short phrase or quotation that pays tribute to or honors a dead person. Another reason why The Waste Land is not an epitaph is because the poem does not really honor anyone or any group; Eliot's poem addresses death and loss, but does not really seek to single out any one individual or group to recognize.