The term "mecha musume" was first used for a series of figurines produced by the japanese company Konami. The figures personified old military vehicles, such as tanks and planes, as cute girls. It's not used to refer to a general art style in which the features of historical military warcraft, or more fanciful science fiction mecha designs. The term means, literally, "mecha girls."
"I love this new mecha musume show. Badass military hardware? Uber-moe cute girls? Why not get both at the same time..."