Despite Mao-chan's 26 episodes I consider this this show as good in-between break for what else you might be watching. It relies heavily on parody elements or it would just be another simple and boring kid's anime. Mao-chan is not outstanding in any way, but it will provide you with a smile, chuckle or even a laugh from time to time, enough to keep most people watching from start to finish.
Animation: Probably the weakest link of the show, it's very simple in the drawing style like many of the other animes aiming for a younger crowd like eg Pokemon. Robbald mentions the episode lenght of 15 minutes, in fact it's only 12:30 min running time, and if you cut away the opening and ending it's hardly even 10 minutes. That turns out to be a bit of a problem with all the reused frames in each episode. The girls' transformation sequences, and also the "defense barrier" are reused every time.
Apart from that there is done an average job on both the CG's and the scenery as a whole.
Sound: The sound is actually pretty good for the biggest part. The opening song is average, but the ending kind of catchy, at least to me. I didn't find myself skipping it more than a few times :)
The girls have _very_ cute voices, maybe they are a bit overdone for the parody's sake but cute is still the best way of describing them. As for the rest of the characters, you will find that they too have very stereotypical voices.
Story: What story? Well basically we follow 3 8 year old girls, who have been chosen to protect Japan from kawaii Aliens trying to invade or rather steal important landmarks. Mao protects from the ground, Misora from the air and Sylvia from the sea. They can't use real military forces to defend because it would seem like mocking the aliens, so they have to defeat cuteness with cuteness! As said the episodes are only around 10 minutes, but with 26 of them it still amounts to something. Still the story never gets more advanced than what can be solved within a single episode.
In many ways Rikujou Bouei-tai Mao-chan reminds me of Alien Nine with 3 girls having to defend against aliens, and also here we got an extremely clumsy main character who is crying several times per episode. We also got alot of similar weirdness, but that's for yourself to discover. The biggest difference is that Alien Nine is much more mature in its overall appeal.
Character: Needless to say, the 3 main characters are very cute, and since this is a parody they are even younger and much more pathetic than your usual high-school heroes. It's funny to see just how useless mao-chan is, and even though the other 2 girls aren't much better it's usually their job to fill in for her. Other characters included in the show are the 3 competitive chief of staffs, the incredily beautiful and intelligent second in command who never gets any credit, the 2 gorgeous cat-eared female aliens, and not to forget the alien chief with the water hole ontop of his head. All characters are as shallow as they can be and already after a few episodes you realize that there will be no development in this department either.
Value & enjoyment: Even though the show has its funny moments, there is no rewatchability. It's just the same stuff over and over each episode, by the end it gets pretty annoying.
In the end I'll just repeat what I've already said: Mao-chan works best as an in-betweener, don't write it off completely, at least it IS 100 times better than its "sister-show" Love Hina. Regarding the overall low ratings, what can I do, I'm also watching full moon wo sagashite right now.
Now prepare for a whole lot of Ahh-ohh-ehh-ohh!