As far as I know, stretch marks make their unwelcome appearence when the skin stretches too fast for your body to keep up with it, ie with weight or muscle gain or pregnancy. They also turn up a lot in puberty, because of growth spurts.
The skin has three layers, the middle layer is the dermis which is a fibrous tissue. When the skin stretches too much, the growth of the dermis can't keep up and the fibres stretch and break. When they break, the blood vessels show through from underneath and this is what you see when you get stretch marks and why they show up red to begin with. Eventually the vessels move back and the fat which is underneath your skin shows through, and they show up as a silvery-white colour.
Hormones affect the production of stretch marks, some produced during pregnancy (sorry don't know which ones) apparently increase in order to make you more flexible in preperation for birth. Perhaps these hormones are the same as the ones partly responsible for the stretchyness in hypermobility? Just a thought.
Dosen't Marfans affect the skin's elasticity, so people with this could arguably be more prone to stretch marks too? Afraid I don't know enough about it to comment.
I have them all over my thighs and oddly over the back of my knees too. My boyfriend has massive stretch marks all over his back which we call 'tiger stripes', because he grew so fast as a child. No hms in his case though. I've never really found anything that helps them unfortunately, now I have a much thicker skin and am less concerned about wether people think my legs look good
It's not specific to hms and my wee explanation is just my guess as to how it could be related.
Sorry for the essay