What are you using to get it into an STL? I've had OK results with DEMto3D plugin, but it has some weird artifacts that I can't seem to get rid of. Are there any better options you're aware of?
I actually use DEMto3D. It's touchy, but I do post-work on the .stl/3d model in blender so it works out ok for me.
If you have weird artifacts, I'm guessing that is due to the underlying data vs QGIS itself. Have you looked at their documentation (https://demto3d.com/en/)?
