I'm sorry, I'm not following. You'll have to explain to me what dying her hair blonde has to do with her acting skills.That's almost like saying Rupert Everett & Nathan Lane shouldn't be cast in straight roles because they gay.
Erm - not really. This is a physical thing, not a mannerism thing.
Alba couldn't pull it off. At all.
I'd argue it's mostly because she's a bad actress, but finding an actress who actually looks the way the character has been presented would have probably helped.
For me, it's a distraction. If she at least looked the part, I believe audiences may have been more forgiving of a bad performance and it's also possible that an actress who looked the part might have had an easier time finding the role for herself. YMMV, of course.