Ferroelectric semiconductors are promising for multifunctional electronics, yet their typically low carrier mobilities remain a major limitation. Using first-principles phonon-limited transport calculations for monolayer In₂Se₃, we show that this limitation depends critically on the microscopic origin of ferroelectricity. In displacive β'-In₂Se₃, low-frequency ferroelectric modes dominate carrier scattering, with additional contributions from longitudinal-optical (LO) phonons, limiting the room-temperature electron mobility to a few cm²/(V s). By contrast, in fractional-quantum ferroelectric α-In₂Se₃, ferroelectric-mode scattering is absent because polarization arises from discrete lattice-scale atomic displacements rather than soft-mode condensation. Transport is therefore dominated by LO phonons, yielding a room-temperature mobility above 70 cm²/(V s). Carrier doping further screens long-range electron-LO-phonon interactions and raises the mobility beyond 300 cm²/(V s) at experimentally accessible densities. These results establish that fractional-quantum ferroelectricity can decouple robust polarization from strong intrinsic carrier scattering, offering a route toward high-mobility ferroelectric semiconductors.