First of all, I have to say: I love mashups. I find them absolutely hilarious. I can be entertained for hours scrolling through the Mate a Movie gallery on Worth1000, giggling at the mental images from the mashed-up films, without worrying about whatever deeper implications for the future of the creative enterprise they may hold. That being said, though, I've heard several people saying that they think the mashup is generally only used for comedic purposes - and while I love the comedy, people are using the technique of mashing up hitherto unrelated videos or music for a lot more than humor.
One - genre, I guess - of mash-up that I haven't seen anyone mention here yet is what's known as vidding - essentially, making your own music video from clips of a movie or television show put to music. Some of these vids are definitely made just for the comedy, but many of them also aim to create a fairly serious commentary on the source material through the use of the mixed media of song and video clips. A friend of mine is a vidder, and she's a lot more eloquent about why she enjoys vids than I could ever be; fortunately, she graciously gave me permission to link to a blog post she wrote about it here, thus saving me from having to plagiarize her words. She also gave me permission to link to one of her vids as an example. (This particular vid happens to be set to Boulevard of Broken Songs, which several people were talking about before, making it a triple mash-up - and one thing she wanted me to mention was that she decided to use Boulevard of Broken Songs because of the way the songs related to each other when taken as a dialogue.)
Although the more serious vids aren't necessarily meaningful/interesting unless one knows the source well enough to recognize the footage, I still would call it a valid - if legally dubious - form of textual criticism when done well. To me, at least, that signifies that this mash-up media has a significance outside of pure humor.
Not that there's anything wrong with pure humor.