"become flatter" is correct and usable in written English.
You can use it in a sentence when you want to describe a decrease in something that was previously more curved or raised. For example: "As the months passed, the hills gradually became flatter."
Be aware that the crown or curve of a structural member such as a floor joist or ceiling joist should be up, so that if the board should sag, it will become flatter, as opposed to becoming more bowed.
As the columns grow larger, their surfaces become flatter and more unstable.
As Burtynsky increasingly makes use of aerial photography, his images have become flatter, stranger: visual puzzles.
The road out of Hpa-An runs alongside wooded hills before the landscape begins to become flatter.
But to the south, toward the more arid interior, the plains become flatter and are protected by a veneer of stones the well-known stony desert with its mantle of gibber (hammada, serir, and desert armour).
The bustle followed the decline of the crinoline, another skirt-shaping device, in the latter half of the 19th century, as the crinoline changed to become flatter in the front and more-emphasized in the back and designs focused on a bunching up of material behind the waist.
Colour becomes flatter, the paintedness of Delaunay's forms more acute.
As he drifted upward, his images became flatter, stranger: visual puzzles.
It was a relief when, south of Salerno, the countryside became flatter, more open, and less densely populated.
The bridge was heightened, the sound post and bass bar were thickened, and the body became flatter.
On the drive there, up California State Route 14, past Edwards Air Force Base, the land becomes flatter and browner, and signs begin to pop up about God, mostly the vengeful Old Testament version, along with ominous Northrop Grumman billboards showing only an image of a dark gray military drone and the slogan "Securing the Globe, 36 Hours at a Time".
