Yes - "Buy low, sell high" would apply to Acorn similarly to any other brokerage account.
Acorn is just a simple investing account where you can buy a limited number of Vanguard's low cost ETFs. The Vanguard ETFs typically are index funds which track a given market index like the S&P 500. The market's indexes are currently lower as you alluded to in your question.
There is no difference in investments between what the Acorn app offers and any other brokerage that trades Vanguard ETFs. The main difference between Acorn and other brokerages (eg: Schwab, Fidelity, TD Ameritrade, etc) are the limits Acorn imposes on your portfolio. The limiting differences are: limited selection of the type of funds you can purchase, the funds are exclusively Vanguard/iShares, and the ratios in which you can structure your investment account are fixed by Acorn's team. You can read about the different portfolio options here under "our portfolios".
Thus, Acorn's investments follow the same ebbs and follows as the greater stock market. Acorn just simplifies what investments choices you have available to you as a DIY investor. Yet, even with it's limitations, it still puts together a simple, yet comprehensive portfolio based on your selected risk tolerance.
If you're interested in knowing what limited selection of funds Acorn offers, check out this link.