This is my take. In isolation, using an obscure character for the Silver Surfer instead of the mainstream version wouldn't be a big deal especially as it's probably the part is relatively small in the movie. But the situation is that the MCU has been having diminishing returns - for various reasons - and they need to put as strong a foot forward as possible. The Silver Surfer is not an A-list character, but he has some degree of popularity and I think there would be a decent interest in seeing the character "done right" compared the rather underwhelming Rise of the Silver Surfer movie. I know the 90's Silver Surfer cartoon while only lasting one season seems to have a really good rep and following among comic book/Marvel fans.
So, I view this move as a silly unneeded risk with little upside. More representation is good, but is a bad way to do it. I'd say if they wanted to have a female herald of Galactus then they should either use Nova or they should just make a new character rather than go with Shalla-Bal. The way that they seem to be doing it (admittedly, we don't know the specifics, nor could leaks be entirely accurate) is unnecessarily pouring gasoline on the fire of the critics of Marvel and Disney and is financially idiotic for a company struggling with their film's box office.
Now, maybe this will all turn out to be inaccurate and the actress is simply playing Shalla-Bal, the love interest of Norrin Radd and Norrin will me the regular (male) Silver Surfer. And if that's the case, I take this back because that would make sense and be reasonable.