The speaker looks at a charming, attractive, wealthy man and wishes to be him. Over and over, the line "I wanna be that guy" repeats like a nagging thought that won't go away. Each stanza adds to the picture: the crown, the admired body, the compliments, the mansion, the big ego. But the speaker also asks, "Who am I anyway?" and ends up calling himself "just nobody," which shows how small he feels beside this imagined figure.
The mood is wistful and a little bitter, with real self-doubt underneath the longing. In the last stanza, the tone shifts from desire to reflection. The speaker admits that envy comes from insecurity, and that it's a "toxic" feeling that is hard to shake. The closing idea is that a person's view of things can't change unless they first change themselves. So the poem is less about the admired guy than about the speaker's own sense of not being enough.