No, it isn't illegal to assault a stuffed monkey - luckily for my dog, who has murdered several - but it is illegal to assault someone with such a monkey, although you'll probably get away with it unless the victim is a police officer. Please make a note of it.
According to the Des Moines Register, a 24-year-old man was arrested late Saturday after an altercation at a "Git-N-Go" convenience store. He may have thought he was following instructions, since the name does suggest you can just go git some stuff and then go - it does not instruct one to pay for the items one gits. (This might have seemed entirely plausible to him if, as his mugshot suggests, he was completely baked.) According to the clerk, the man left with one pair of sunglasses, two packs of smokeless tobacco, and not one but two stuffed monkeys.
Unfortunately for him, the clerk was able to flag down a nearby officer, who was quickly able to locate the one person in the immediate area carrying one pink and one orange monkey. The officer questioned the suspect, who allegedly then became "aggressive," and when the officer tried to take him into custody he "allegedly swung around and hit the officer in the face with a stuffed monkey." That got him charged with assaulting a police officer in addition to disorderly conduct and fifth-degree theft.
Those are relatively minor offenses, but I'm guessing there is more to the story, most likely a probation violation. I base that on the clerk's claim that, when she asked him what he was doing behind the counter, he said, "Don't you worry about it. I'm going back to prison anyway." Predictions like that usually come true.