Naah, 13 episodes seemed just about right for the anime. Having read the entire manga, there's very little that could be added or expounded with regards to Midori and Seiji's relationship. The anime seemed to wrap it up quite well.
If you still want more MnH goodness, though, then I suggest you get your hands on the manga. Lots of other wacky, weird, and sexy characters are there and there's even a funny character "development" on Kouta!
