What settings are you using to export your video? I would recommend starting with setting the export dimensions to 640x480, and then using one of the Presets, such as the "Best Quality, Fast Encode, 29.97 FPS" (and uncheck the "Use Frame Blending" option). Note that you can export just a little bit of your mix by dragging the ends of blue time slider/region at the top of the Export window. After you export with these settings, try playing it back on your computer and see if you are happy with the quality.
