While I agree that the artwork itself is kind of boring, I attribute that more to the artist than design in and of itself; it's just symptomatic of modern BA art direction where the shading is oversimplified for the sake of a unified aesthetic, when in some cases some characters could use more individual shading tweaks as per the design.

That being said, the design itself is completely sound and appropriate for her character. It's obviously supposed to be a "pure" and "angelic" design that's befitting of Mika's status after all of her character development. The total lack of pretentions in the design - hair completely down, no more accessories on her outfit, and her outfit itself is extremely revealing - as some have suggested it's closer to lingerie than it is a swimsuit; she's practically naked. All of this is simply alluding to a sense of exposure, a feeling of "baring" her heart to Sensei and her friends with no more walls between them and her. Mika is just a girl, after all.