Is it safe to use a 3D printed icing smoother?
As mentioned, FDM 3D printed parts are flawed for food service and prep use regardless of the material, because of the small gaps between lines where bacteria can grow, and because your printer isn't used in a way to keep it from introducing contaminants into otherwise clean material.
However, there are some ways around this.
Single-use
I'm pretty comfortable using my printer to make items for one time use. If I wanted an icing smoother with a fancy shape for a special cake that I'm not likely to need again soon, I'd go ahead and do that. I would apply all the icing at one time, and then I'd discard the piece, rather than try to clean and save it. Also note that I'm not talking about a commercial kitchen; this would be for a cake I'd eat myself with friends, rather than sell.
The big thing I've done this way so far (I've had my printer less than a year) is make shaped cookie cutters. I'll print the cutters, use them, and then throw them away. If I want the same shape again some time, I'll re-print.
Lining
The other thing you can do with food prep items is print them with the intent to use liners. For example, here is a 3D-printable taco train, where a train car has grooves to hold  tacos (yum!). It wouldn't be good to put a taco directly in here, but you could use napkins or similar food-safe liner to separate the 3D part from the food. In the case of the icing smoother, you might be able to print the part and wrap it in aluminum foil.