When he's not static, i.e. playing on the right in a 4-3-3, he's much more dangerous. 3-4-3 suits Mahrez a lot better coz he can be positioned more centrally due to overlapping RB but can also drift out wide and interchange positions with KDB and an overlapping RB.
The problem for him when we play 4-3-3 is that in this system our RB overlaps very rarely, thus, he's often left in static positions to combine with KDB and subsequently free up KDB who whips a cross in. Exception was probably Leicester at home where Chillwel gave him an acres of space to exploit and he was good even with nobody around him to combine with. When he's isolated and double marked on the right in a 4-3-3 his dribbling is too one dimensional (step overs and always cutting on his left), thus, he struggles often.