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.
At this point, comics are a storytelling medium like novels, movies, or TV. Nobody notes that "certain movies" contain audible dialogue just because films were once silent.
Here.