Though your response to this question is completely subjective, I would agree with Madgett's statement. I, too, in my initial readings of the poem, did not recognize its potential irony. Perhaps you did not either.
Notice the words that Wheatley emphasizes, presented in italicized form in contemporary printings of the poem:
'Twas mercy brought me from my Pagan land,
Taught my benighted soul to understand
That there's a God, that there's a Saviour too:
Once I redemption neither sought nor knew.
Some view our sable race with scornful eye,
"Their colour is a diabolic die."
Remember, Christians, Negroes, black as Cain,
May be refin'd, and join th' angelic train.
The concept of Wheatley's West African homeland as "pagan" is a Western conceptualization of any non-Christian, polytheistic society. The belief among many Europeans of Wheatley's time was that, by enslaving Africans, Europeans and Americans of European descent could "civilize" them. In fact, the justifications for slavery and conquest were that Africans and Native Americans were "heathen" peoples who were not worthy of respect due to their ignorance of Christ. Wheatley seems to address this belief when she writes of how her "benighted soul" was taught "to understand / That there's a God, [and] a Saviour, too..."
The next lines address the Biblical scorn with which black people were regarded, as though they were in possession of the mark of Cain. The last two lines read as an aphorism, or a lesson. She addresses "Christians," but the italicization can read as either a plea for understanding or an expression of frustration -- or both.
"Christians" and "Negroes" are separated only by commas. In one's reading of the poem, depending on the pace and rhythm, one might be inclined to join the clause "Remember, Christians, Negroes, black as Cain," instead of reading them as separate clauses, as in the following: "Remember, Christians: Negroes, black as Cain...". The inadvertent conjoining of "Christians" and "Negroes" allows for the possibility of their being one and the same.
Joining "th' angelic train" reads, on the surface, as the conversion of Africans to Christian faith. More deeply, it suggests the possibility ("may [my emphasis] be refin'd, and join...") of Africans assimilating into Western culture. Arguably, one's blackness does not preclude one's ability to be just as "refin'd" as any white person.
A more militant reading could perceive Wheatley as saying that Africans, if given the chance, could become like white people, in every aspect but skin color. However, this assumes that the poet is expressing her own ideas about the African experience in America, whereas she could be expressing, with great irony, the view of many white Christians who wished to convert slaves.