I've never actually seen a black person be mad about a white person doing their hair in dreadlocks

Agreed, I have never seen anyone of any race making such argument against any other race.

using that to reject the whole idea which does have some merits

I cannot see the merits at all. The whole idea of some races being more entitled to clothing/hairstyle/music/food etc. is ridiculous and inherently racist.

While the comic you posted may be almost fighting a strawman, it is not wrong in criticising the idea per se, just possibly criticising something quite niche and rare.