Herd. A group of animals who have a collective life together. Or a crowd of people.
The cartoon is making fun of people who, like sheep, copy everyone else or "follow the herd".
Herd mentality is when people do the same things at the same time. For example, buy an Ipod, or go to the same coastal resort in the same car.
Of course, it's human nature. Who wants to be the only one wearing leg-warmers when everyone else is doing cowboy hats?
But being different is definitely more fun. Like Jenny Joseph says in her fabulous poem, don't wait till you're an old woman to wear purple. :-)
Make fun. Subject to laughter or ridicule.
Follow the herd. Do something because most other people are doing it.