Is Montego Bay in The Beach Boys' "Kokomo"?

No. The Beach Boys’ 1988 hit "Kokomo" lists these Caribbean destinations:

Aruba, Jamaica, ooh I wanna take ya
Bermuda, Bahama, come on pretty mama
Key Largo, Montego, baby why don't we go
Down to Kokomo...

The lyric is "Montego" (referring to Montego Bay, Jamaica), not "Montego Bay" in full. So Montego Bay is referenced—just shortened to "Montego" for the rhyme scheme.