The first time I saw Comic Sans font used in a mathematical lecture was when I attended a talk by Peter Winkler in or around the year 2000, and I was immediately struck by the font's refreshing insouciance. (In saying this I realize I sound like a font snob, along the lines of the wine snob in a Thurber cartoon who says "It's a naive domestic burgundy without any breeding, but I think you'll be amused by its presumption.") More recently, I attended talks by Avi Wigderson and Fan Chung that made excellent use of the font.
Meanwhile, it's gotten overused in high school students' homework assignments and Youtube videos, to the point where some font-afficionados feel like banning the font entirely. But I still like it, and my on-line lecture notes on stochastic processes (developed for my UMass Lowell night-course 92.584), written in the form of Mathematica notebooks, make extensive use of the font.
I contend that, when it comes to a lecture topic like math that in the wrong hands can be arid and forbidding, it's a good countermeasure to adopt a font that from the start announces "Let's have some fun."