it is *good* to allow this username if used in satirical contexts. the more fun is made of hitler, the better it is.
dont glorify his actions or support those who continue his mindset/vision, but dont ban everything related to the name and the person, because this behaviour will result in forgetting.
and forgetting what happened is the worst that could probably happen.
this account is clearly satirical and does only make fun of hitler as a person. it is not used to spread his believes or to harrass others, it is not used to relativize his actions - therefore its fine.