A customer of ours has sent me a file as an .stl format from Google Sketch, it opens fine in Solidworks but looks messy with lots of lines. I want to be able to convert to sheet metal and flatten the part but the option isn't available. Is that the end of the road for this type of file or can it be worked with? I've attached a screen shot of what I'm seeing.
No. STL is mainly for 3D printing. Its a wire mesh format. It will be faster to redraw.
Those triangulated face may not be flat/co-planar due to rounding error.