Learn Hardcore Korean: jigeum gibuni johayo.

이 is the subject marker attached to nouns ending in a consonant. Here, it tells us that 기분 (“mood/feeling”) is the subject of the adjective 좋아요. If the noun ended in a vowel, you would use 가 instead.

Could I use 기분은 instead of 기분이 in this sentence?

Yes, but the nuance changes.

기분이 좋아요 simply states “(My) mood is good.”
기분은 좋아요 highlights or contrasts the topic “as for my mood, it’s good (but …).” You often see it paired with a contrasting clause:
- 기분은 좋아요, 그런데 일이 많아요. (“My mood is good, but I have a lot of work.”)

What’s the difference between 좋다 and 좋아하다?

좋다 is an intransitive descriptive verb (adjective) meaning “to be good” or “to feel good.” It takes its target as the subject:
- 기분이 좋다 → “(My) mood is good.”
좋아하다 is a transitive verb meaning “to like” someone or something. It takes its target as the object:
- 음악을 좋아하다 → “to like music.”
  In speech, you conjugate 좋다 as 좋아요 and 좋아하다 as 좋아해요 in polite style.

Where does the adverb 지금 fit in, and what does it do?