Learn Hardcore Japanese: yama no kesiki ha ii desu.

は marks the topic of the sentence (“as for …”), so 山の景色は means “as for the mountain scenery.” By contrast, が marks the grammatical subject or highlights new information. Saying 山の景色がいいです would simply present “mountain scenery is nice” as a neutral fact, while …はいいです emphasizes it as the topic you’re talking about.

Why do we use いいです instead of a noun plus です?

いい is an i-adjective meaning “good” or “nice.” In Japanese, i-adjectives act as predicates by themselves, and you add です only for politeness. So 景色はいいです literally means “scenery is good.” There’s no need for an extra noun-plus-です construction.

I thought the dictionary form of “good” was よい, not いい. Why do we say いいです?

Historically, よい is the classical form, but in modern spoken and written Japanese, いい became the standard way to say “good.” Note that when you conjugate いい, you use the よ stem (e.g. よくない for “not good,” よかった for “was good”), which reflects its irregular nature.

Why is there no separate verb like “to be” apart from です in this sentence?

In Japanese, i-adjectives like いい already contain the meaning “is ….” The copula です just adds politeness. You never need another “to be” verb (like いる/ある) for adjectives.

Can we omit です in casual speech, or is it always required?

In informal contexts, you can drop です:
• 山の景色はいい (“The mountain scenery is nice.”)
You can also add casual sentence-end particles:
• 山の景色はいいね (“The mountain view is nice, isn’t it?”)

Could I use another adjective like きれい instead of いい? What’s the difference?

Yes. きれい（な） is a na-adjective meaning “clean,” “pretty,” or “beautiful.”
• 山の景色はきれいです = “The mountain scenery is beautiful.”
いいです simply means “good/nice” in a general sense. Remember that with きれい, you treat it as a na-adjective, so you don’t say きれいい—you say きれいです.