Joe is carrying her a couple of times in the background of Zoe's Easter vlog. Not remembering her birthday is a little strange, but, and I am sorry to stereotype, he would not be the first man to forget a family member's age or birthday. (I really hope I don't come across as sexist - obviously it's not every man, and equally women can forget too. But I speak from experience: my dad is that uncle. All my cousins on my dad's side are much older and were born before my parents met. We have no idea how old they are/when their birthdays are because my dad cannot remember and he never wrote them down.) Also, with Joe not having his own kids, he wouldn't necessarily know where children should be developmentally at each age. I certainly wouldn't.
I think he's just awkward, both in himself and around kids, and makes up for it by trying to be funny, which, in turn, makes everything even more awkward. Some of his earlier interactions with both his nieces and Dianne's niece are incredibly cringeworthy to watch. He appears to be slightly more comfortable now...
I mentioned it on the old thread, but Vlogust has really highlighted how much/little Zoe sees of certain people. Considering her and Joe are supposedly so close, both personally and geographically, they appear to spend very little time together, especially as the vlogs with them both in the thumbnail do considerably better.