I'm trying to discern why one would make the comment "not funny" about a web comic. (I see this a lot, so it's not just you.) If the comic's purpose isn't necessarily humor, why would it matter if it's not funny?
Obviously many webcomics have other-than-humorous intent, but I would like to point your attention to the word "comic" and its original meaning... maybe this is why there is a tendency to note that certain comics are not funny, even if we acknowledge that this is not their intended purpose.
Here.