Introduction of hippos into the South would probably be pretty bad all around in the long-term.
Given that they would be introduced with the intent of establishing sustaining populations, the lack of animals really capable of predating upon hippos (other than people) would likely result in high birth rates and a reasonably successful establishment in the South. Given that the Colombian population originating from Escobar's escapees has grown and spread quite rapidly - which numbered far fewer individuals than a planned introduction would result in, and started from a far smaller origin point - it is reasonable to assume at least a similarly rapid radiation of American hippos would follow.
Past that? Farming hippos is not really going to be commercially viable, so that will be abandoned. Furthermore, people are going to find hippos far harder to hunt successfully, and far more dangerous, than any native game animals. Their utility will rapidly be discredited, and public opinion will be liable to sour alongside that. In spite of this, I don't see attempts to cull the new population as being successful. Hippos are far more cryptic than any fully terrestrial large mammal simply on the basis of being able to submerge for hours at a time, and keep in mind that there will likely be at least a couple decades' worth of population growth in the rural South - where penetration of culling efforts is likely to be minimal at best - before efforts get serious. If anything, the hippos probably expand outward at a decent clip, though to what extent is hard to fully determine.
I don't see hippos becoming a truly horrible agricultural pest, but they will certainly be a nuisance in that regard. Their reputation as highly dangerous animals will grow rapidly with them, as will their reputation as messy, environmentally destructive beasts. Nonetheless, the South has gained a new pest, and one that is unlikely to leave anytime soon.