Code needs precision and emoji are not precise - they display wildly differently across devices and there's no universal agreement on what they mean anyway.
We've got 👊 over in the sidebar which I presume means 'fist bump' but I've also seen it used for 'punch'.
Someone commented on some code of mine once with a grinning cat emoji and it looked like an angry cat to me. They were confused when I asked what was wrong with the code. So yeah it adds emotion to text, but not uniformly-understood emotion.