It both is and is not. Printing the mold is not the problem. Silicone rubber is really expensive, unless it’s for high value parts, such as medical or extreme industrial environments.
Much cheaper is to buy off the shelf chinese TPU whole made mecanum wheels. I did that for the practical hands on experience, else I would just buy an 8-inch mecanum wheel that can easily hold 40 kilo each. The real challenge was the 3D modeling of the entire assembly. Making it was trivial.
You can buy 1 ton mechanum wheels for industrial robots with the motor and suspension if you really want to go heavy duty. This is a very mature technology.
Cheers,
Valentine