I can't say anything about the t-shirts under dresses thing - it is definitely "a thing". It's a bit 90s, but if that's your vibe I don't see anything wrong with it. There are plenty of magazine articles about pulling if off as a look. The fact that Carrie doesn't seem to understand how to put colours together is more the issue imo.
WRT Joel following baby accounts I find it a bit odd. I wouldn't be comfortable with an adult man who is unrelated to me following an account with pictures of my kids, so I don't put their pictures online. Bit sexist of me, I'm sure, but I find it generally odd to follow a baby account, especially if you're a childless adult man. When you tie that together with Joel sharing Tom's baby announcements before they knew each other I find the whole business a bit... unusual. Wouldn't go so far as making any wild, dark accusations but it makes me feel slightly uncomfy.