This was one of the first movies I ever got to see in theaters, and for nothing else it's a nostalgia trip. I must have watched it a lot as a kid cause I remember it pretty clearly, without having seen it recently.
The script was a cheese factory, and the characters are pretty silly, but it was never meant for adults. I personally as an adult found the princess to be entertaining, but mostly I credit this movie, and the series from which it was made as being very positive for young girls. Rainbow Bright, while being a bit silly, was a strong female presence in network after network of male oriented cartoons intended for the same audience.
What ever sillyness and unrealness came from the movie are secondary to me. It's a nostalgia trip, and a good memory to boot. Show this movie to your young female children. The boys won't like it, but I think that most young girls will still see what I saw in it almost twenty years ago.