AP Human Geography time!
Overpopulation is sometimes a tricky concept, as it can mean many things in many different situations. Often, it is a situation in which there are so many people in one place that the city is unable to provide adequate food, shelter, water, and sanitation services.
Because of this definition, there is no specific population density that is considered the boundary of overpopulation. Even if two places have the same population density, one place could be more overpopulated than the other, based on the availability of resources.
To answer the title question, cities are not necessarily overpopulated just because there are a lot of people. If there are enough resources, then it is not overpopulated.
On the other hand, most cities create a massive influx of poor or unemployed individuals looking for jobs. Also, a lot of wealthier people move from the cities to the suburbs, since commuting to work is now much more possible. This creates a situation in which many cities do not recieve enough taxpayer dollars, and this can lead to the decline of the inner city.
I’m not sure if I have answered your question, but I feel that I’ve talked about some relevant stuff.