In my experience it is a certain class of 'designers' that mock Comic Sans. These 'designers' exist a plenty in the wild even though they are not really 'designers':

1) Have Apple branded computer - check.

2) Do not know how to add content to a CMS - check.

3) Incapable of making the smallest tweak to CSS - check.

4) Move files around with a DropBox type gadget and have to get a 'developer' to 'FTP' that PDF to a server - check.

5) Only 'design' for Mac screens with a minimum resolution of Full HD even though the iphone has been out for a while - check.

6) Take inspiration from the latest site-du-jour in a me-too copy-and-paste way, bringing no blue sky thinking to the table - check.

7) Think in terms of CMYK and DPI at all times - check.

8) Never use client content in mock ups, go for Lorem Ipsum when shapes on a page are needed - check.

9) Throw a tantrum if a developer or client makes the slightest change from the cast in stone design mockups for a good reason such as usability - check.

10) Mock 'Comic Sans' - wow I am a real designer now!!!