Yikes, sorry, just now getting to the forum after a long summer absence. I can give you my opinion, which is only just that, but based on having skied with many students in the intermediate range.
Oh and a huge shout out to those of you who ask my to chime in. Makes my day to have some hopefully helpful information for fellow ski fanatics!
Do we have endless mellow blue terrain no. However, while probably a tad steeper than most, our blues and double blues are nice, long, almost all groomed and seldom too crowded. It is not true that the blues are cat tracks. However to get from the upper mountain (Tram - black only, and Sublette - one double blue) you will indeed have to traverse using cat tracks. Most of the blues and double blues however are off of Apres Vous, Casper, Kemmerer, and the Gondola, and there are definitely non cat track options for getting down to the base if double blues are ok for you.
While steeper than many resorts, in my fifteen years teaching, I have only really had two or three guests that would have been better served if we had had more gentle blues. Everyone else quickly improves and finds that with a lesson or two they become very comfortable managing their time on the mountain if they had any issues to begin with. If anything, I think that Jackson is a great place for riders/skiers to realize that they can enjoy more than they ever thought they would and suddenly they go from intermediate to thinking about off piste, trees, etc....
Let me know if there are any other questions I can answer for you. I hope you choose to come and don't regret the decision. I would be happy to provide you a list of my blue progression (which can vary based on conditions of course).