There's an element of "I wouldn't have started from there!" unfortunately.

As Doddy says, the STL model is built from a series of flat planes and the CAM will try to reproduce that. If the mesh is fine enough, it may be that CV mode in the motion controller will smooth these out, with a bit of tinkering with the motion controller parameters.

You cannot, mathematically, generate a spiral with G2/3 codes as the radius of the curve is changing continually. Best you can do is a series of close-spaced circles, gradually moving up the cone. That lets you use G2/3 commands. However, you do need a circular model with smooth curves to start with. Personally, I would use F360 for something like this but even then, it often generates many short G1 moves where you might expect curves - but the motion controller seems to smooth them out.

The free version of F360 will import .step, I believe.