I think at least part of the reason Danton's physical appearance is mentioned so often is that he himself referred to it in a speech, which is pretty unusual. That said, I think Marie-Hélène Huet is right in arguing that Danton, like Robespierre, has been turned into a kind of "revolutionary monster" by historiography and fictional portrayals alike: the "minotaur" to Robespierre's "sphinx," as she has it.

As for Mme Roland, I honestly can't remember whether she mentions Danton's appearance in her memoirs and I haven't read her correspondance, but even if she does, I doubt it had quite the influence it's often claimed to have had.