I think some photos would help us see what is happening.....I am a bit confused. Are you using any batting yet or is this problem just in the stitching of the strings together.....one or two layers of fabric doesn't provide much middle for the stitches to met in. what you are seeing may just be the hole and shadow created by the needle. sometimes stitches will look like the bobbin thread is coming up to the top....but once the piece is washed...this disappears....I always like to somewhat match the top and bobbin threads in color and weight....as to your question. yes you will see more of the top thread on the bottom of the fabric if your bobbin thread is too tight....there are a lot of good video's online if you just google or search for thread tension problems.....