For whatever reason, birds in general are not seen as being very smart. I'm not sure how a species that regularly misplaces car keys and TV remotes can refer to a scrub jay that can remember the location of six thousand items (acorns) as a 'bird brain", but we humans routinely use terms like "dodo", and "dumb cluck" to refer to people who don't seem very bright. The last term, "dumb cluck" seems directed specifically at chickens and one objection people raise about chickens is that they are stupid, dumb birds. And, admittedly, chickens may not beat any corvids (crows and jays), parrots or even pigeons when it comes to bird IQ.
But recent research suggests that chickens are nowhere near as clueless as some would prefer to believe they are. One researcher tells audiences about remarkable animals that "understand sophisticated intellectual concepts, learn from watching each other, demonstrate self-control, worry about the future, and even have cultural knowledge that is passed from generation to generation". Audiences apparently assume he is talking about monkeys, but those are all behaviors witnessed in chickens.
Now, as so often is the case, it turns out our inability to notice evidence of chicken intelligence may have more to do with our limitations than the chickens. Research at the University of Connecticut suggests chickens have at least two dozen distinct calls communicating different types of information that they have been using for millennia, while we are just getting around to trying to learn what they mean.
The next time someone tells you chickens are dumb, find out if they are basing that on personal experience or just repeating what they have been told. My guess is that people who have low opinions of chickens either haven't spent much time with them or only experienced confined white leghorns, which have limited ability to demonstrate their capabilities.