Does Yoko Littner Wear A Bikini Just For Fanservice???

Yes and no. The bikini is indeed for the fan service, but also because other types of clothing would restrict her body movements, as her rifle is equal to that of a Hecate rifle. As fighting with primarily her rifle requires her to dodge gunman attacks and "quick-draw" her rifle to shoot at joints that are only visible within a short timeframe, jackets and conventional pants would most likely hamper movement. Not to mention that if they were to get caught on a branch or a scrap of metal that was shot off, she'd be a sitting duck until she freed herself.

Yoko Littner Bikini

It's so that the show will get views