Simple:
* As grumpy as a bear with a sore paw.
* As grumpy as a wet cat.
* As grumpy as a thundercloud.
More descriptive:
* As grumpy as a cactus in the desert.
* As grumpy as a troll guarding a bridge.
* As grumpy as a teenager whose phone died.
* As grumpy as an old man who lost his dentures.
* As grumpy as a teacher who just graded a stack of failing exams.
The best simile for you will depend on the context and the level of formality you are going for.