Is it strange for a 13 yr old to want to have thin thighs?

No: Many 13 year olds wish that one body part or another looked differently than it does. Society sets unreal expectations of how women's bodies should look, which can make puberty, when areas like legs and thighs get bigger in all women, a traumatic time for some. Focusing instead on what your body can do (running, swimming, etc.) can help keep a positive body image.