I think the modern success of MF would not be possible without the current designs. As is, I thought they were kind of a nice nod to the currently popular steroetypes and also what I call the more old-fashioned proportions used in older anime. (If you look at other shows airing in the last 3-4 years, there's a disturbing lack of style and distinction... to the point that characters are exchangeable.)
When people who don't know the show see doodles from there, they really immediately grasp the personalities based on current anime sensibilities because it is rooted in modern sensibilities. At the same time it's not so generic like the last two GUndam properties which went so far to reach to the younger audience that everyone has big eyes and no mouth.
(I also think the designer did a great job in separating out who is beautiful, cute, pure, etc. That's not something other animes tend to care much about, but this one I felt was really good about making characters distinct on their visual presentaiton and not just what they did in the show.)
As much as I like Mikimoto's aesthetic, I don't know that the infusion of new fans would have happened with his stylizations.