Love these new technical terms!
My experience with mashups are quite recent and from a personal perspective rather than work. I used Zazzle, which interfaces with Flickr, to make postcards for my baby's first birthday invitation. I even had postcard stamps made with his picture on them. Fun!
I think the trading card maker is very simplistic. You can't edit much, the space is limited... I just don't see great applications for it because of the limits. That being noted, we did include some basic trading cards for primary age students in the new Primary Gifted Program Curriculum.
I think the magazine cover has greater potential for gifted kids because of the article titles. You can get more depth out of this application. It could be a fun, quick product for a research project on any topic.
I see the mosaic maker as a modern collage, without the overlap. Collecting all the images would take a great deal of time, but it could be used in the same way. Selecting symbolic images can be a high level activity and the ensuing discussion would be rich.