Most mammals do. Eating mammals could very well be a mistake. I prefer birds and fish, but it seems a shame to kill them, anyway, and not eat them. There are plenty of people who still eat mammals.
... besides, that is one of the Greenpeace lines, calling the meat "bad" to keep people from eating them.
It's not specifically mammals that are problematic, but organisms which are high up the food chain, since pollutants are magnified by an order of magnitude or more for each step you take up the food chain. Thus swordfish, shark, dolphins, orca, sperm whales, seals, polar bears etc are all bad, but cows, deer, tilapia etc are all fine, assuming there's not an acute pollution issue in the area they were raised in, or where their feed was produced.
I know you are right (most mammals are near the top of their food chains) and it has been quite a few years, since I had any dolphin, but there are people who eat them, anyway. They don't care about things they can't see, because some of their people are near starving. When Greenpeace goes after some perceived wrong, it is usually the poor guys, trying to support their families in a third world country who are most often harmed. I get disgusted with this when it happens.
Sorry, but I don't put dolphins above humans, like some of the animal rights activists do and I'm not advocating lining our livers with lead, mercury, polonium, thallium, bismuth ...
MODDED to remove Emo Bullshit and typo.