It's a dystopia in a less obvious way than 1984 is. It's a vision of society where nothing matters because everything is permitted and everything is great.
Entertainment is fantastic, the soma drug is plentiful and makes any situation, any self-reflection on the emptiness of life totally fine, every person is born into a class and they are conditioned to be happy to be of that class, whether on the top, middle or bottom of society.
Everyone has sex with everyone, technology is well developed. It's a high-tech society to be sure, but ultimately a soulless one.
Think a lot of dystopian novels deal with the idea of oppressing people by stopping them from thinking (Fahrenheit 451 is another example). So even though a lot of people in Brave New World are 'happy', they are stopped from thinking which is a form of imprisonment. I think that's what makes it a dystopia. Or controlling them what they think.