There are two main meanings to this cartoon.
First, there is the issue of bicycle helmet use. The use of helmets by bicyclists is somewhat controversial. Many people feel that they should be required to prevent injuries. However, other people say that requiring them reduces the number of people who are willing to bicycle. This is said to harm the environment. It is also said that the aggregate health benefits of bicycling will outweigh the aggregate harms from head injuries. The cartoonist here agrees with the latter view. The cartoon implies that it is fine to bike without a helmet.
The second meaning is that bicyclists are more responsible than those who drive unnecessarily. The unseen person in the car has, we are told, just driven his large car half a mile simply to get coffee. He could have walked or biked, but he did not. The cartoon implies that this sets a bad example. It is saying that it is important to get exercise by biking. It also implies that it is more responsible to bike than to use fossil fuels unnecessarily.
To me, the cartoon is less more about the irony of giving advice when your perspective is so narrow.