I was going to reply to this this morning but then I had a migraine and didn't want to form words if I didn't have to.
anyway. I have not thought much into it until Cyndi asked the question but I think it is a fairly good choice. I haven't seen mads Mikkelsen in many of his things. I mean I have seen doctor strange but can't really remember his character. I have seen his brother, Lars Mikkelsen, in a few things, notably ride upon the storm, and he was really great so I have faith that Mads will be just as good.
the choice of a danish actor is an interesting one. first and foremost I am very much a daneiophile. so on a personal level, I am happy.
I think I would suit the character as from what I am aware/have interpreted Grindelwald went to durmstrang which is somewhere in northern Europe (given the heavy uniforms and talk of flying over snowy mountains) and services northern and eastern Europe (seeing as the Bulgarian Krumm attended) so it kind of makes sense that Grindelwald is from northern or eastern Europe (and unfortunately most English speakers wouldn't be able to differentiate and pinpoint accents much more than northern or eastern).
Although i haven't watched the Hannibal (and don't intend to) from what i can tell from interviews and clips Mads has experience in playing the charming man who is twisting the way others see him to his own ends, or should i say the twisted man who will do whatever he wants to and using a charming facade to get others to like him.
and that response is sort of longer than expected so i will leave you with a cartoon from my favourite webcomic about the role of mads in Dr strange.