My brother complained of the same thing. TBH I just didn't have time to change them. The trash cans were actually baked into the building models and I just added the springboards on top. But KayKit's models pull from a color map png and the separate surfaces aren't editable. I could have rebuilt them, but we were on day 10 when he made the comment and I was coding until the deadline. (My last upload other than bug fixes was 1 minute before the deadline.)
I did consider changing the red cars in the first two levels, but I was trying to use them as a psychological aid. They were all placed so that if you jumped right before them you could get the candy or land on the trash cans, and then in levels 3 and 4 jumping in the same location would cause you to catapult. So it was an attempt at psychologically training the player in the game mechanics.
But you're right, I'd like to change them going forwards.