I don't think that the occasional fanservice shot necessarily undermines a strong "Action Girl" portrayal.
Also, how the fanservice shot is executed can be pretty important. If it's a passive sort of thing, where a skirt just flips up or breasts bounce a bit while the girl is in motion, then that's not necessarily a problem. If the scene draws massive attention to it by having male characters gawk and shout like fratboys over it, though, then it starts to feel a bit sexist to me.
Basically, if the male characters have a tendency to treat a female character as a sex object, then the narrative is kind of implying that this is how the viewer should approach her as well. But if the anime just quietly winks at the audience and throws out some "eye candy" then that's not necessarily a problem. A lot of it really does come down to execution.
Personally, there's a certain threshold for me, a certain point at what a specific female character is thrown into fanservice
so often (and/or with such little subtlety) that I can't take her seriously any more. I would say that Code Geass' Kallen is right on the line for me there, as I could still take her seriously, but it was a struggle at times. So any female given
less fanservice than Kallen was, I wouldn't have a problem with it. Any given
more than what Kallen was, then I'd start to find it a bit sexist (or at least objectifying).
With respect to Obelisk ze Tormentor's point, I think there's something to be said about a female character that can seem strong and sexy without ever needing a pervy camera angle to get there. That's much of what I take from his comment - Haman Karn is so strong that the anime never felt the need to sex her up.
Mind you, Gundam (especially the older ones) don't have a lot of conventional fanservice anyway. I don't recall many fanservice shots of Sayla, Relena Peacecraft, or Lacus Clyne, either.