The term "baby daddy" is oftentimes used as a derogatory term. For a man it means they provide no financial support or help in the raising of the child. For a woman it means they became pregnant (purposefully, accidentally, or through traumatic sexual violence) and their child does not have an active father figure parent.
Most of the time I've heard the term used when a woman has several children from different partners. When the term is offered it usually is meant as an insult, slut-shaming the woman or implying the woman is of loose morals.
There are times I've seen where a woman has a relationship and becomes pregnant, but then the partner bails on the relationship or tries to get the woman to have an abortion. If the woman makes the decision to keep her child and the sexual partner refuses to participate in raising said child or supporting them financially, the woman may decide to call the partner a "baby daddy" instead of a father in order to place distance between her new family unit and the former partner.