Toppers were released in the 0.9.0 update and were the last type of kart customisation added to Smash Karts. 

Most toppers are attached via a stick to the back of the kart, in between the exhaust pipes. The stick changes colours as you choose different kart skins. Some toppers will be attached to the kart without the stick, and instead are attached directly to the kart (e.g. Big Brother). 

Toppers are available for Coins and in the Spinners. So far there have not been any toppers in the Season Track.