there’s nothing inherently wrong with fanservice so long as it isn’t losing focus on what the character is about. someone like femspy, for example, i could see using sexuality to her advantage and so her wearing fishnets and a head scarf doesn’t immediately strike me as unnecessary. another thing to keep in mind is the era they grew up in. Medic wouldn’t be wearing a skirt because it was an attractive thing to do, she’d be wearing it because that’s just what she’d wear. not to say they’d strictly adhere to that, but it opens a window of plausibility. for the most part there aren’t any femmodels that strike me as over sexualized EXCEPT for the femsniper but since she is an original character fulfilling honestly one of the least involved in battle class roles, i’m gonna leave that one alone (with whatever internal issues i might actually have with it).
as for dressing for WAR… remember that tf2 is a very silly game. i don’t think war is an appropriate word to describe what they do (unless you’re Soldier) because if it was there wouldn’t be any hats and they’d all be wearing uniformly 100% practical outfits. i mean for godsake, Spy is running around in a three piece suit and dress shoes! and skirts can’t be that bad on the battlefield if Demo can do his job wearing a kilt.
fanservice is a tricky line to define because you want the characters to bring their personalities with them into the fight, but you have to be careful not to take it too far.