I am so glad to see sewing theme parties on the rise! Here is older, but still fresh!, Frosting for sewing themes here.
...here are a few more!
Source and tutorial
Add some drizzles of chocolate to your cake balls... not only will they look like yarn balls - but it will cover up any imperfections ;o) Source
...another pop idea... these spools of thread! Source and tutorial found via maddycakemuse