It's illegal because it may cause mis-identification (I wonder if such word exists?) of the car. Let's say if a car with converted looks is involved in a hit and run case involving a person/motorcycle/car or maybe it is used to commit a crime. It may cause confusion to those who witnessed the incident.
But to tell the truth, most Malaysians can identify a Wira straight away even with full front convert to Lancer or Evo looks. In fact, even if they saw a real Lancer CB (the one Wira is based on), they'd probably thought it is a Wira too.