Are dolphins smart?

Not particularly.

They can't even actually communicate with any sophistication:

A disproportionate amount of dolphin research time has been devoted to teasing out any potential for language – the science-fictional myth of dolphinese – from their vocalizations. If dolphins had language, we would almost certainly have found it by now. When their vocalizations turned out to be rote and inflexible, “I’m scared!” “I’m mating!” “I see food!” pretty much covers it, the research turned to echolocation clicks. Perhaps dolphins were sending each other 3D holographic messages encoded in their clicks. Nope.

They do have signature whistles that identify the dolphin as an individual, but that’s the most referential thing about their communication system. In contrast, ground squirrels have an amazingly semantically-rich signaling system. Nothing about the dolphin whistle repertoire would prevent it from being used as a discrete combinatorial system to convey unlimited meaning, it’s just that dolphins don’t use it for that.

Also, they are jerks:

Adult male dolphins routinely kill porpoises, not for food — or even out of competition for food – but because the porpoise is similar in size to a dolphin calf. The killings serve as practice for their regular infanticidal behaviour, a sure way to ready mothers for mating.

Interesting review/summary of a book that's coming out in a few months.