Saggy Sail?

My sail has a sag in the foot and no obvious to me way to tighten it up? Is this the design of the sail? Is a better sail flat?

Sail shape determines how much lift the sail produces. Adjusting the shape is how you "tune" the sail for current conditions.
In general fuller sails have more lift, but more drag, flatter sails, less lift, less drag.

It is the balance of forces, hull shape under the water, keel shape, amount of rudder deflection, sail shape and balance that makes your boat perform the way you want.

That is perfectly normal. When the wind fills it, it builds shape in the sail, which is what produces lift. For recreational sailing, there is no real need to tune the sail for conditions, unless I suppose you are going sailing in maybe 15 knots plus when maybe you'd want to tighten things up to flatten and depower the sail. That said, I bet 99.9% of Sunfish that are not used for racing, and probably 50% that are used for racing have never, ever had their outhauls tightened or loosened. BB

That is the normal design, there is extra draft cut in the foot, moreso in the newer sails.