How likely do you think it is that the Frey antipathy towards Crannogmen comes in part just from the poverty of Crannogmen leads to many of them to steal cattle and other livestock.

The crannogmen are never mentioned by the Freys as cattle-thieves, despite a long list of insults. In terms of illegal activity, the only thing they mention is that the crannogmen might harbor outlaws. 

Based on the historical political economy of the fenfolk, I would guess that the thing that the Crannogmen get up to most that pisses off the Freys is smuggling. Remember, the Freys make their living off of charging tolls for people trying to cross the Green Fork – if instead, you can hire a crannogman to get on his boat and take your goods through the Neck and around the Twins, that’s money out of their pockets.