I was wondering, if one was to make a plastic (read as carbon fiber or
fiberglass) is the layup the same as trying to get plaster smooth?

I find the 2 very similar, but the sheet rock is a whole lot easier to sand and
you don't have to be precise. It just has to look good. This is true of
fuselages but wings have to be "right".

Don't let that scare you away from a glass plane. The work required is really
not as much as most claim - if - IF - you do good work from the foam up.

Same as sheet rock. If you have to cover up a poor foundation you have more
work.