To mutter, grunt, grumble, and is what a grump does.
A grump is irritated by everything that is not "just so". With an eagle eye for hairs to be split. Fussing over the details and snorting in disgust as faults are found.
When applied to work that needs doing: This is a valid stage. Some things do need to change.
To be grumpy about people is wanting them to change.
That is where being a grump stops being useful and starts being a problem.