Gonna have to say yes, only because you're young and the outfit will probably attract attention of people older than you and they'll assume you're also older. I can see how your mother might be a bit protective of you because she doesn't know all the strangers at the cons and let's be realistic there are plenty of creeps.
I think you could edit the costume to be a one piece bathing suit instead of a bikini top and longer shorts, maybe if you cover more skin, your mother might go for it?
Originally Posted by Lithium Flower
Once upon an evening dreary, while I lurked weak and weary,
Over many a quaint and curious volume of forgotten posts,
While I nodded, nearly napping, suddenly there came a bitching,
As of some one gently pitching, bitching at my chamber door.
`'Tis JasonTerror,' I muttered, `bitching at my chamber door -
Only this, and nothing more.'