I agree. None of the letter sounded like it came from Ramsey.
It sounds to me as if SmallJohn is trying to bait Jon into attacking Winterfell. If my theory is correct on the Manderly story being wrapped into the Umbers, then it would set up a perfect double-cross. If SmallJohn could get Jon to attack Winterfell, it would provide him (SmallJohn) the perfect opportunity to turn on Ramsey and help Jon from within Winterfell.
Top that off with Litterfinger seemingly about to call the Vale to the north to help John....I think Ramsey is screwed by seasons end.