I thought Mermin was adorable. He looked more like an aquatic frog crossed with the Creature from the Black Lagoon than a mercreature. He was curious and hyper, which often caused a lot of unwanted trouble for his human friends. The story itself was very short and to the point. There wasn't a lot of plot development or twists and turns. It was very straightforward.
The pictures were nice though. The entire book is in color and very bright. This would be very appealing to younger readers. I think there was an even mixing of pictures with text-- some pages even had just illustrations. The story was easy to follow and smooth. There was no sense of being choppy with the formatting or layout.
If the cover doesn't make it obvious, this is best suited for the 10 and below crowd. Die hard graphic novel fans in the 11-12 year old range might give it a try, but older than that is not likely.