Technically...no. Not legal. But I seriously doubt anyone would bother prosecuting a single person defacing a couple of bills. The law is there to stop people from destroying currency to a point that it could affect the value of that currency of the market. Magicians have been quite openly tampering with currency for decades and i have never heard of anyone getting any trouble for it. On this site you can buy gimmick/gaffed coins and bills that have been cut, drilled, bent, etc. and I doubt penguin has got into any trouble for it.
Reference:
United States Code
TITLE 18 - CRIMES AND CRIMINAL PROCEDURE
PART I - CRIMES
CHAPTER 17 - COINS AND CURRENCY
§ 333. Mutilation of national bank obligations