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.