Is the United States of America a mosaic of various cultures?

Your concept is quite up to the mark, you just need to focus on the ethnic blend of USA while describing the colonization campaign. Make sure you don't get diverted as it annoys the examiner when you get distracted from the main topic. So, my suggestion regarding this question would be a three point approach:

1) In the beginning give a thesis statement answering the aforementioned question in affirmative

2) Now you have to prove your thesis statement by writing about the advent of people belonging to different ethnic and religious backgrounds, and How they got amalgamated into the US culture?

3) Conclusion