Some mounds and barrows have flat tops. Mostly from brave adventurers digging it up to get the loots.
Here's a picture of Uppsala Högar (Uppsala Mounds)
Here's a bronze age stone mound from Ekornavallen close to my home
Here's Anundshög with a flattened top due to grave rob.. eh.. adventurers.
Here's Grönehög with more recent (192
excavations
Well, you get the picture. All those are a couple of thousand years old and erosion has done alot to flatten them. Newer mounds should be rounded according to the experts.
I don't like the mounds under the obelisks. They don't blend well with the heightmap. If you want them to blend better, try putting some stone slab under them, or some sort of plinth or something. Maybe a ring of smaller stones?