I'm late getting to this, but I have a few to add.
I'm blanking on which episode it was, but I liked it when he was dancing in front of the steps and Bloo joined him. He has a big smile and is kicking his feet and it's very cute.
In "Crime After Crime" Bloo goads Mac into doing the worst thing he can think of. The worst think the little kid can think of is jumping on the bed. And is still well mannered enough to take his shoes off before doing it.
Speaking of which, I always like when he gets a wardrobe change. The main thing I remember about "Cookie Dough" is that he wears a hoodie instead of the usual tee-longjohns combo. And that his square head is visible through the hood. This goes for outfits that are meant to be sweet, like the "Bride to Beat" tux, and those that weren't, like the Rambo outfit in "I Only Have Surprise for You." I even thought it was cute in "Partying is Such Sweet Soiree" when he didn't have an outfit at all!
Speaking of which, I like his bare-bottom baby pic and his sour reaction to Eduardo finding it in "Infernal Slumber." And that his jammies are blue (just like Terrence's are red.)
I like when Wilt tickles him and Bloo in "Make Believe it or Not." It's nothing big, but it's really charming.