Somewhat clunky writing, kids didn't like the illustrations
Kids! The bar is high with them. It's tough, I think, when you can get Caldecott-winning picture books from the library; kids aren't going to settle for drawings that look like stills from a generic PBS cartoon.
The color scheme was vibrant but not particularly balanced, which made some of the scenes veer into garishness. The fact that it was digital illustration didn't help: there was no texture to soften the pieces.
Beyond that, the color scheme in the owl's face didn't allow for a clear enough contrast to readily read his facial expression. It wasn't unreadable if you sat with it for a second, but if a kid's first reaction to the third page of a book called "Grumpy Owl" is, "he doesn't look that grumpy," then you've got a problem with how you're rendering the face.
More frustrating to me, though, was the composition: often the protagonist is looking over his shoulder to chastise the other animals, but they don't seem to be able to hear him. They are never depicted reacting to him at all. So the ending, where he writes apology letters, seemed strange, because it didn't seem like he actually hurt any of the other animals' feelings in the first place; rather, he just talked behind their backs...to himself.
Also, the owl has a kid? That seemed to come out of the blue.
Finally, the writing was okay. It wasn't super compelling; I'd much rather read Dr. Seuss or 'Chika Chika Boom Boom' to kids if I have to read a rhyming book. Whereas this book, all of the rhymes were predictable, and some lines seemed to be clunky or tacked on just for the rhyme's sake.
That said, the writing wasn't bad. I think the manuscript might have had some potential in the hands of a better illustrator. The fact that the Owl kept interjecting with a single word, like "False," was funny, and the repeated phrase, "Alright? Good day?" was clever. More of the owl's curmudgeonly personality is shown in that single phrase than in a bookful of pictures of him.