You mention proper feeding and mental (I'm going to assume you mean social) stimulation as if they are minor details. They are crucial to a child's development. Parental neglect or poor feeding in the first 3-9 months are going to leave their marks on a child.

Language development is a good example. Here's a review going over language development in the first year of life. https://pubmed.ncbi.nlm.nih.gov/26756156/ Imagine a baby who is fed and clothed, but who is deprived of a parent who directly talks at them. They're going to have hard time picking up verbal and nonverbal language. Their language development might get delayed.

A toddler who has difficulties understanding the language that he is expected to understand is likely to get frustrated, which in turn may lead to behavioural difficulties. This may lead to them becoming unsociable and other children won't want to play with them. Pediatric psychiatrists deal with this all the time!

Now, you don't need a PhD to raise a baby. They're not going to be traumatized because you said the wrong word or didn't buy the right brand of diapers. Feed them, talk to them, change their diapers, etc. They're probably going to turn out fine. But to claim that the first months are basically irrelevant (that's how I interpret "pretty much clean slates") is just plain wrong.