YES. Many people assume that it's illegal to stamp or write on paper currency, but it's not.
Basically, there are three things you CANNOT do to USD paper currency:
You CANNOT change the denomination — for example, you cannot add two zeros to a one dollar bill and pretend that it’s a one hundred dollar bill. That’s illegal.
You CANNOT burn, shred, or destroy currency, rendering it unfit for circulation.
You CANNOT advertise a business on paper currency. For example, if you own a Bagel shop, you cannot stamp “Eat at Joe’s Bagel’s” on a dollar.
But don’t take our word for it, see what US Law has to say on this topic:
“Whoever mutilates, cuts, defaces, disfigures, or perforates, or unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, or Federal Reserve bank, or the Federal Reserve System, with intent to render such bank bill, draft, note, or other evidence of debt unfit to be reissued, shall be fined under this title or imprisoned not more than six months, or both.” 18 USC Ch 17: Coins and Currency – §333. Mutilation of national bank obligations
“Whoever designs, engraves, prints, makes, or executes, or utters, issues, distributes, circulates, or uses any business or professional card, notice, placard, circular, handbill, or advertisement in the likeness or similitude of any obligation or security of the United States issued under or authorized by any Act of Congress or writes, prints, or otherwise impresses upon or attaches to any such instrument, obligation, or security, or any coin of the United States, any business or professional card, notice, or advertisement, or any notice or advertisement whatever, shall be fined under this title.” 18 USC Ch 18: Crimes and Criminal Procedure- §475. Imitating obligations or securities; advertisements
18 USC 333 is written to prohibit the malicious destruction of currency, and 18 USC 475 is written to prevent currency from becoming a vehicle for commercial advertising, like for Burger King. Because the people who stamp their bills with a portrait of Harriet Tubman want their stamped money to stay in circulation and are stamping to express their opinions about a political issue, not to make a profit, they are good to go.