I think you should make the cake. If it's just a regular cake that you usually make.

I don't think you should be required to make a cake and paint the statement "KKK 4ever!" or "NAMBLA rules!" on it.

When it comes to your own personal art and craft, you should be able to refuse any design. No one would have an issue with a landscape painter refusing to paint a portrait, or to paint in a different style, or with different media. It's their artistic choice. I see painting a slogan - or taking specific request for design that you may not want to do - as part of that.

As a comparison: let's say the baker was asked to make a "joke cake" with a chocolate turd on it. The baker doesn't want it to end up all over social media tagged with his business name. Does that make him a humourless piece of work? So be it. It's his choice what he does with his brand.

If a baker doesn't want to make a cake with "gay weddings rule!" on it, or whatever else, that's his choice. Does it make him a bigot? Likely. But it's choice to have a bigoted brand.