Is this a valid hypotheses - dinosaurs became extinct because mammals ate all the dinosaur eggs?

Not likely, while it could be a plausible explanation if it had just been the dinosaurs that went extinct it could work. However, the event that killed off the dinosaurs also killed of a variety of other organisms both on land and in the ocean, where mammals had not yet established themselves.

A hypothesis that explains the extinction of the dinosaurs, but not the other life forms that died out in the same event, is incomplete.