To find out how much more Mary's kitten weighs than Lynn's kitten, you subtract the weight of Lynn's kitten from the weight of Mary's kitten:
\[ 86.23 \text{ ounces} - 67.74 \text{ ounces} = 18.49 \text{ ounces} \]
So, Mary's kitten weighs 18.49 ounces more than Lynn's kitten.