I agree, if you look even at another example of a "sub-unit" group, like say Orange Caramel, they had a very clear concept and stuck to it the entire time. You never saw them come out in all black leather outfits or something one week, it was always some cute fairytale dress or something that fit the song. Or even look at another "rookie" group like Secret or Sistar, both kept with clearly defined concepts as well and saw a lot of success.
I think 5dolls has a decent and catchy song, but they don't know what they want it to be, the music video is kind of long and depressing, and only features 2 members, and then their performance outfits are kind of all over the place as you mentioned, nothing really meshing with anything else as central to their concept, or giving any hint to what it actually is.