I think the reason I feel a little ehhh about the pairing is that Jamie is a little kid. Since my imagination isn't good whenever I stumble upon a fanfic for them I always picture Jack doing things to a little Jamie. But with all the fanart of older! Jamie i'm starting to warm up to it more
I guess that does help a little. And yes, I'm a crazed Hijack shipper XD I kind of like the idea of Jamie being a reincarnation of Hiccup btw. It truly gives new purpose to the meaning "love never dies."