As I started to buy some more, er, fitted jeans I am having or bordering on having this problem. I think it is a function of fit and fabrication with the lycra and especially any poly blend with the cotton. The very tops of my thighs are a bit chubby and so even with a recent pair of "trouser" jeans, that are not at all skinny but do fit kind of snugly through the hips, they are tighter in the very upper thigh than would be ideal.
My conclusion is that it may be hard to get the "jean fit" and a truly fall-from the hip slightly looser upper thigh fit--even "curvy" may not do it. I have a pair of EB jeans that somehow do skim just perfectly but they need a belt to hold them in place, although I could take them to be altered and see if that helps--so not truly BF but not butt-hugging.
I also was feeling a bit timid about tucking or otherwise emphasizing the thigh-torso junction. I think any new jeans I buy will have to be even more trouser-like. I know that's not the same thing for you wearing skinnies.
I don't know how much of this crease is "acceptable," but I know it doesn't take much for me to feel self-conscious.
