Yes, I fully agree that line weights in Sketchup would be a very handy feature. And while we are at it we should also be able to set the color of the lines.
Let me explain why… Even though we can set line weights in Layout it would be much faster and easier to do it inside Sketchup itself. Layout is still slow and clunky when working with tags and it would also make a lot of sense to have the same settings (dash, color, lineweight) in both Layout and Sketchup. This way we could set most of the line style inside sketchup and only use Layout to override this settings where needed.
I’ve made this suggestion about 4 years ago already unfortunately it hasn’t received any love yet… (see here Suggestion for 2020.2 new linetype feature improvements)