No far from it. It is just drawing the right shape at the right size in the right place. The fact i have an engineering bend rather than an artist flair helps.
A large proportion of, and all the “thingyverse customisable” items are Openscad. Have a look at About PPLLab - Thingiverse for some of the type of thing i tend to create all very simple really.
I think it is unbeatable if you want to change an existing stl. (Remove something, add a logo or name, alter holes/pins, scale and incorporate as part of another design etc)
Like this General 3D printing questions and answers, tips and tricks - #123 by PPL