I'm having the same problem with my cheapie Brother C-100. When I zig zag the bottom threads come up and mingle with the others, even after I've tried changing the tension. Other stitches, except the straight ones, have more or less this problem.
With straight stitching it works nicely, straight and looks good, back and front. Got so that I finally started putting the bottom and top threads match so it won't show on top. I'm not too happy with that, so right now I'm hand appliqueing and not doing stitching by machine so much.
Can't find the manual that came with it, I'll have to download one from the Internet.