Depends on whether you are in China or Japan ;-). All kidding aside, 米国 is only standard in Japanese, maybe acceptable in Chinese as jocular or internet writing, but not on your HSK :). My 5c, Aurelio

汉语美国; 日语米国

Ha, thank you for that ^^ Also i think you mean "homophonic" which I'm not entirely sure is an actual word, so your autocorrect put homophobic, which means a dislike of gay people.

You know , to add a syllable of [i] to the end of a word can make it sound more soft and crumby. Such as doggy piggy As same as in Chinese : 美国 米国 米 is homophobic with 迷(second tone) One of the 迷's meanings is 迷你= mini-