I'm not quite sure what audience Mirror/Mask was aimed at. If it was made for kids - it's rated PG, I think it's too ugly to interest many kids. If it was aimed at adults, I think the story is too simple and, to be quite honest, rather dull to interest them. For a fantasy, it was rather unimaginative when you consider the plot. A girl fights with her mother because she is growing up and wants independence. The mother falls ill shortly afterwards and the girl feels responsible. She works through her guilt and her own conflicting emotions about growing up in her dreams. I'll give the production team full marks for attempting to create a surreal dream world. It looked really, well, complicated to put together. But it didn't have any magic. Alice in Wonderland and the Wizard of Oz did it far better, and those were made decades ago. Just goes to show you that all the special effects that money can buy can't replace a great story, interesting characters, and good acting.
Another thing that I thought was missing was a sense of peril. For some reason, none of the trouble spots that Helena and Valentine got into seemed very dangerous. The only there was any real sense of danger what when Helena and Valentine were going to be eaten by the sphinxes. But that was all resolved before they even really started to do anything but surround them. Maybe if they had at least licked their lips I would have pretended to be concerned. But, alas, they didn't and Helena and Valentine easily escaped. Then again, maybe I'm too jaded and know that she will eventually have to wake up so she can't really die in her dream.