They are orthogonal, plenty of people are happy with themselves despite being abusive or toxic. It’s a poor heuristic for one’s ability to socialize with others or be a parent. Also, unhappiness can be something people are in denial about.

I think what people are trying to say is that if you want to become a parent you should be capable of providing the child with a good childhood. That’s an admirable goal but answering that question is exceedingly tricky.