I'm not sure if "cultural appropriation" is the word for it, but she's not wrong, not fundamentally. It's not uncommon, on certain parts of the internet, for people to adopt a sort of "sassy black person" manner for comic effect, and gifs can play into that.
Now, I'm of two minds of the "blackface" characterisation. It seems unfair, because the image of black people in popular culture is a bit more complex than just some musical buffoon; most of these gifs play on a perception that black people represent a greater emotional authenticity or self-possession than white people, and their use by presumably white millennials represents not a confidence in white supremacy but a disenchantment with the psychological and emotional enfeeblement of the white middle class. On the other hand, the characters played by black minstrels were not simply buffoons, but could be witty or sly or charming after their own fashion, and were similarly identified as emotionally authentic, but that did not make those portrayals any less derogatory.
I think the common thread as the depiction of black people as naive: both of them represent a view of black people as basically child-like, as just doing or saying whatever comes into their head. While the former may find some romance in this where the latter views it with contempt, neither are humanising depictions, both of them reduce actual, living black people to a mirror image for white people to define themselves against. And that's enough of a concern to at least think, for a moment, whether the gif you had in mind is appropriate.