Low-temperature mobility spectrum of two-dimensional electron gas (2DEG) at the inverted interface of AlGaAs/GaAs quantum well has been evaluated theoretically taking into account nonabrupt composition profile due to segregation of Al atoms into the well. In this approach, the Al content profile at the inverted interface has been considered as exponential decay function, and transport mobility components were calculated in the Lindhard's framework and flat interface approximation. It was found that alloy scattering due to segregated Al atoms can be significant, and even limits electron mobility at high 2DEG densities. The segregation decay length is evaluated from comparing experimental mobility spectrum with theory.