Lilac eyes. Lean face. Dragon rider. Those are all Valyrian traits.  Jon Snow has none.

The much younger Rhaenys died in the sack on kings landing at 3.  She looked like a Martell.

There is enough evidence of Valeryan traits being recessive to keep alive the theory that maybe Jon Snow's father is Rhaegar.  I think GRRM might not even know - he seems to like to leave lots of things open ended.