No, it is not. Mountainous is an adjective meaning of, about, or like a mountain. The adverb form, mountainously, has very rare usage.
mountainous
The noun mountain has the adjective form mountainous. The adverb form is "mountainously" (whether pertaining to an actual mountain, or metaphorically).
Mountainous is an adjective.
it is mountains
The terrain was mountainous.
Iceland is mountainous.
Yes, South Korea is mountainous.
Yes! Most of all Kosovo is mountainous !
Mountainous Croatia's population is 90,643.
Switzerland is noted for its mountainous terrain.
The root word of mountainous is "mountain."
In the USA, there are several mountainous areas. For example, Nevada is known as the most mountainous state. In second place is the state of Alaska.