It depends what your interests are. There are plenty of public buildings of archetectural interest that date back to the 16th century when the Spanish arrived, and the remains of various Aztec temples have been well-excavated in recent years. The museum in the large park is world-class and covers the anthropology of Aztec, Mayan and other tribes from the pre-Spanish periods really well. They are a fantastically hard-working people who will trade anything to anybody. But there is nothing like the range of options of things to do as in NY.
We spent 2 nights there at the start of a longer holiday - could have easily occupied 4 days, but not longer. Hotels and hostels are very very cheap because the labour rates are so low. The city smells polluted at all times unless a strong wind blows because there are so many cars. Mexico is a huge country, there are scarsely any trains but a comprehensive long-distance bus system get the population from place to place.