Depends on which place you are talking about. Manpower is a blessing but excess of anything is bad even if it is a good thing. Too much of it could make it bad. Having huge manpower but not being able to use it in every field will make it a weakness. In many countries, there aren't many opportunities. Thus the competition is high. The government is unable to provide its population with work. In cases like this, it will be a weakness for the economy.
On the other hand, if your country can provide you with good opportunities, overpopulation won't be a problem. Despite having competition, everyone will be able to contribute to the cause. That's how this will become a strength of the country's economy.
But if you want to talk about the whole world's economy, overpopulation helps a lot. Everyone alive needs to consume food. Poor, rich, middle class. All of us need food. So as we consume food on a daily basis, we are helping the economy.
Cheap labor is based on work experience and also certificates of expertise. The price of a job is cheap. because it does not require expertise to make policies and decisions. the thing that can be done to get to reduce the price of a job is to take advantage of technology.
if the workforce in question is related to the public, such as social services. it can not. It's different if the work in question is related to technology, it could be replaced with a machine. for example in agriculture. The country can make the concept of automatic irrigation for example.