If something is sagier, it means it is more saggy or droopy.
Saggy things are not as strong or firm as they should be.
Examples:
The old mattress is sagier than the new one.
Her baggy pants are getting sagier.
The dog's ears are sagier than they used to be.
The balloon got sagier as the air came out.
The bridge got sagier and sagier until it fell down.
The old pirate's pants were so sagier, they fell down!