Illusions don't use costumes
It’s a known bug reported since the day the patch hit. Lots of threads were created in this board, in the bug board and ingame. And still we don’t even know if Anet has plans of fixing it.
Pumpkin – Mag
Oh well.. Guess we wait and cross our fingers once again.
The same is true of the Selfless Potion halo effect. Not as obvious, but still sucks.