This study investigates the upper critical solution temperature (UCST) behavior and the coupling between crystallization and phase separation in blend systems composed of poly(ε-caprolactone) (PCL), poly(lactic-co-glycolic acid) (PLGA), and the zwitterionic monomer DMAPS. The results show that DMAPS exhibits a dual effect on PCL crystallization and a pronounced plasticizing effect on PLGA, reducing its Tg from 44.2 °C to 25.7 °C. In the ternary PCL/PLGA/DMAPS system, DMAPS functions as an ionic compatibilizer, enhancing interfacial miscibility while lowering crystal perfection. Overall, the ionic–dipolar interactions introduced by DMAPS simultaneously regulate chain mobility and crystallization dynamics, leading to morphology-dependent UCST transitions and offering design guidance for thermoresponsive, biodegradable biomedical coatings.