Did you use #MOJO_IMAGE_FILTERING_ENABLED=False ? This should work on all targets except html5 in the current version. I did a fix for it but it hasn't been released yet.
Here is the related forum thread.
There is a short hand solution. You could use DrawRect() with a hight/width of 1 instead of DrawLine(). I think I read something about a general Issue with drawing lines in the browser where you have to fiddle with exact floating point position values to get crisp lines and adding to that the thickness of a line is not working with scaling but with rectangles it is.