A flat topped flower cluster is called a corymb or a cyme.
A flat topped mud brick tomb is called a mastaba.
An unusual flat-topped seamount is called a Guyot.
Cyme
ziggurat
the pyramid of giza.
plateau
That is called a Mesa, or a Plateau.
mesas
Butte
mesa's
Guyot
We apply the term mesa to a flat-topped mountain.