No it is indeed not crazy for adults to watch cartoons. Some people believe that people who watch cartoons are immature but it has nothing to do with the maturities of an individual. The answer to that is simple it makes them feel better. 
A cartoon show carries you to a parallel world,where no one could yet harmed and where no problems exist similar to ours.In otherwords watching cartoons is an escape from the reality. Cartoons also help people to feel nostalgic. It reminds them of a simpler time and those good childhood days.
There is no harm doing it, as long as this escape is for a short period of time. Moreover, this is a healthy form of escapement one can use.It is definitely better than drug or alcohol abuse.
To summarise this all, itz totally OK for an adult to watch cartoons.

-Vigya Goel