Is this expected or should I change something to get them to use the whole cage?
The enclosure is based off of seeco's "The Ultimate Cage for Babies Hatchlings Neonates Pygmies Any Small Chameleon"
If they spend a lot of time up there- they are too cold and/or you don't have enough climbing furniture below. My bet is too cold, because from your pic it looks like you have a fair amount of furnishings.
Babies might spend very brief periods basking up there because it is the closest they can get to heat or UVB. But it will only be occasionally for brief periods and unlikely to normally see several up there at once. At least in my care- I might see one crawl up there for a minute or two every now and again, but they don't hang out in group for long periods like that.1
They don't hang out upside down in nature. That should be a big clue to everyone saying this is perfectly normal behavior.