Firstly not all South Indians are Madrasis/Tamilians. We have Kannadigas, Telgites, Keralites and they all belong to the south part of India, but yes Iyers are Tamilians. I am an Iyer too but I have vada max thrice in an year. Also in our custom Vada is never prepared on happy/auspicious occasions like Marriage. We definitely do not eat Idli everyday, that too for lunch. We live on rice mostly, rather lived for even we are turning health conscious and including roti/chapati in our daily life.
But on a different note, Tamil is probably the most exposed South Indian language and may be thus this reference to all South Indians as Madrasis. Also cinematic liberty does allow them to stereotype/typecast people, be it the Punjabis or South Indians.
The contrast is mostly shown between Punjabis and Tamilians. It will be nice if people start experimenting with other parts of India as well.