Yes, it makes sense to sign up. This November, the Supreme Court will hear oral arguments on a case – brought by Republican state officials and supported by the Trump Administration – to invalidate the ACA. However, the Court likely will not rule on this case before the summer of 2021 and the outcome is not yet known.
Meanwhile, people who need 2021 health insurance coverage through the Marketplace should sign up during Open Enrollment. Doing so ensures your access to coverage for the entire year. If the Court were to rule in a way that would render your coverage less affordable or desirable during the year, you would have the option to disenroll at that time.