Personally, I wouldn't chance it but its your tank and your fish, do what you want. Plastic has way too many harmful chemicals that can leech into the water column. I won't risk my fish with anything other than stuff specifically marked as safe for aquariums.
If plants for the aquarium are made of plastic and the toys you buy are probably going to end up in a child's mouth at some point, I cannot imagine that the plastic used in a toy is going to be toxic. I do not see why you couldn't use one. Craig has his Simpson's tank and I don't remember if there are action figures in it but you might ask him.
Don't laugh, but in younger days (when Disney's little mermaid was very popular), I had a couple of the all plastic mermaids and other characters in the tank for a while (about 2 months) and they did no harm (that I could tell), but one does wonder about the paints on the toys - some toys are made of a pre-coloured plastic and others have layers of paint on them and I have noticed that anything with paint on it that stays in water for a while starts to flake and I'm sure that you would not want your fish consuming paint flakes.