A pronounced anisotropy is observed in the low-temperature mobility of a two-dimensional electron gas formed in an In0.75Ga0.25As/In0.75Al0.75As quantum well grown on a GaAs substrate. We show that the mobility differences along [011] and [011¯] directions are mainly due to In concentration modulations. Spatially resolved photoemission measurements show an asymmetric indium concentration modulation, correlated with the surface morphology observed by atomic force microscopy. A theoretical model considering conduction band energy modulations agrees well with the transport measurements. The identification of this mobility limiting mechanism allowed us to design and grow higher quality two-dimensional electron gases, needed for high indium content InGaAs device fabrication.