This is more a feature request, as it would require Principle to support mixed strokes, as Figma does. (or import the strokes as individual rectangles in Principle).
At the moment, using the mixed strokes feature in Figma will cause those objects to import with a full border, making it pretty time consuming to re-draw them all in Principle.
Thanks for reporting this, I fixed it for the next version of Principle: layers with mixed stroke will get imported as images - this is how layers are imported when there is anything that Principle doesn’t support, such as gradients, multiple strokes, etc.
I’m not sure when that update will go out, but in the meantime, you can add “principleflatten” to the layer name of layers that have mixed stroke weights and Principle will import it as an image, just like the next version will.