It must be because of the ownership pattern of rickshaws and taxis in Mumbai. The personalised slogans and art behind vehicles in Mumbai are fewer than in other cities, especially Bangalore. For example here is a collection of Gaadi Slogans from the Garden City collected by Vasuki R. Click here.
Luckily, there are enough images and slogans to read on Mumbai auto and taxi backs to keep you entertained in any traffic jam. I wish that there was more.
Here are some more Gaadi Slogans by Rin Supreme, here. Enjoy!