Not only that the overcrowding and fighting over limited resources causes psychosocial stress, which might explain the mental illness epidemic nowadays?
The number of people being born in very poor countries is essentially irrelevant compared to the consumption choices of people in rich countries. Based on current trends, the global population is expected to peak at around 10 billion, but the planet is comfortably capable of sustaining billions more if we can find a middle ground in resource use between the dire poverty of DR Congo and the wanton profligacy of the US. Talking about birth rates in relation to climate change is at best a misguided distraction and at worst wilful misdirection.