Yes, the costume will come off once your tama goes to sleep. In the morning when it wakes, it will appear as it normally would.
If you can't wait until bedtime, change the time to when your tama normally goes to bed, then chnge it back to the correct time and the costume will be off.
Doon't forget that the nyatchi (stuffed animal) costume is a one-off use item - you will need to pay for it when you enter the code if you want to use it again. The rare hohotchi costume can be used again and again.
GK1