Yoshi has already stated that with the vanity system he doesn't want to see mages in tank armor, or tanks in caster armor. You will be restricted by what your class/job can normally wear, if the item has your job/class listed in it's requirements then yes, you can wear it on that class with the vanity system.

This is totally super old news though. In fact I believe it was explained in a live letter before the game even launched. And I am fine with that.

Of course, you are going to see "Miqo'te in swimsuits" because they aren't restricted (the swimsuits, and really any/all event gear) to any specific class. It really shouldn't matter to you what someone wants to make their character look like using the vanity system. What should still really only matter is if those players are decent enough at their class/job to make the dungeon run a nice smooth and enjoyable experience.