As I recall the main reason it was picked to represent X-Marks is:

The Dandelion is a COMPOSITE FLOWER, meaning that it is made of hundreds of individual flowers joined a common base.
Most people think that is a flower with yellow petals, when in fact each one of those yellow so called petals is actually a complete individual flower joined at a common base surrounded by the green petals.

If you look at each yellow part with a hand lens you can see the Stamen and pistil inside of them.

So in reality the Dandelion is like X-Marks with hundreds of individuals joined at a common base.

Wear you Dandelion proudly