Well. Jamie doesn't really know Dean that well, other than the couple of times they've run into each other, so it's a little harder to say. Dean seems like a good guy, and he knows that he makes Kaylee happy. And happy Kaylee=happy Jamie on general principles, so there's that.

But...there's this little spark of jealousy, too, because he does like Kaylee as something a little more than just a friend. Ship life means he doesn't really have to deal with it much unless it comes up in conversation, so it's generally not much of an issue. And as long as it's non-romantic conversation about Dean, then he's generally fine.

He is glad that Dean was able to exorcise the Darksoul from Kaylee, for obvious reasons, and he's a little curious about it, but...dunno if he'd actually ever ask about it.

Overall, though, he's mostly okay with Dean.