Yes, thickly is indeed an adverb.
Some example sentences are:
She lives in a thickly populated area.
The thickly-clothed man makes his way through the ruins.
Thick is an adjective, but can be an adverb in some cases. In others, the adverb form is "thickly."
the plains are thickly populated
Thickly snouted pipefish was created in 1814.
A word used to describe a noun is an adjective; a word used to describe, or modify, a verb or an adjective is an adverb. Thick is usually an adjective; thickly would be the adverb form. You look at the usage of a word in a sentence to find its "part of speech."
I love hot toast, thickly spread with best butter.
large and thickly populated settlements means
wavy
assam
comparative
Grass
It means to spread or apply thickly as you would jelly on toast.
Because ----