Emoji in math has a useful niche, illustrating basic mathematics for young schoolchildren, and for simple recreational puzzles. Beyond this, the accessibility issues raised by others seem to be a greater concern to me.
My physics teacher in high school would sometimes replace variables with smilies to make a point that the symbol representing the variable is not the thing itself and I think it helped some students who had problems solving the exact same equation with different variable names. Of course naming conventions are important and were mostly used, but the smilies always stuck with me for some reason.
I've done this, too, when teaching remedial algebra in graduate school. I'm pretty sure it helped some people, while not confusing those who were already confused anymore.