Yes,
you must file this income. Mary Kay will send you a Form 1099-MISC, which they also file with the IRS, so the IRS knows.
Because
you earned money, the IRS will not regard this as a hobby. The hobby rules are
invoked when you lose money, and then only if there are other circumstances
(see below). Even if it was a hobby, you are not excused from reporting the
income.
https://turbotax.intuit.com/tax-tools/tax-tips/Small-Business-Taxes/When-the-IRS-Classifies-Your-Bus...
