The word 'comic relief' is being thrown around here a lot. Captain Marvel is retro, sure, and Billy Batson is in fact a kid, but where are you guys getting the comedy part from? I figure he's no more funny than any other goofy super hero in a cape. The difference is, because he's less popular, and because the concept is so rooted in golden age comics, Captain Marvel maintains his 'retro' or classic story and visual trappings. There's less pressure for DC to update him, because most don't know of him, and those that have don't really want him to change. He may seem somewhat silly, but he's merely dated, and intentionally so. Superman gets to change with the times, but Captain Marvel stays the same, and that's fine.
If you want to look at a hero from the same era of comics who also hasn't changed at all, but unlike Captain Marvel, is legitimately played for laughs, look no further than Plastic Man.