Looks plausible (particularly for Monroe.) The percentage foreign born in those counties was small and they create an island surrounded by high concentrations of newer immigrants. While foreign born is an imprecise measure, it gives some indication of Northern settlers as well.
On the other hand, the slave populations were proportionately higher in some other counties in the region, suggesting they were also part of Little Dixie before, although they had more recent immigrants. Looks to me like they were beginning a transformation.