Comic Sans is a terrible font, even when used for its intended purpose. It has a really off-kilter awkwardness that doesn't evoke any sense of youth or playfulness.

The perfect argument against Comic Sans is Chalkboard [1], a font created by Apple to have the same look but fixes everything wrong with it. Vincent Connare, the creator of Comic Sans, uses it as an example of how great Comic Sans is, saying "if Comic Sans is bad why make something similar." [2] But they made something similar because it was bad and needed fixing. Chalkboard shows that the concept of a casual handwriting/comic font is valid, it's just that Comic Sans blew the execution.

[1] https://en.wikipedia.org/wiki/Chalkboard_(typeface) [2] http://www.connare.com/whycomic.htm