It might be helpful to remember that a lot of people do not apply to university straight out of secondary school, for a wide range of reasons. And she has done essentially nothing but school for her whole life. One of the luxuries of graduating from high school early is that she has the time to stretch and release the tension of fitting into whatever it is about school that currently doesn't align with who she is. In her case, developing a wider and more effective repertoire for managing her mental health would seem to be an obvious focus for that time (not that that isn't a focus right now, of course!), and then secondly, formulating some version of her mid- and long-term goals, which includes exploring her passions and joys. She'll go into college (if that turns out to be the appropriate next step) with a clearer vision, focus, and motivation for that stage of her life, which will come across in her college applications and interviews.
I would say that not applying to college of any description is a perfectly legitimate short-term post-secondary plan--as long as there is some kind of plan for that time. E.g., if you want to spend the time deciding if your passion for painting is something to pursue seriously, then good. Take six months to a year to paint and see where it leads. But that requires thoughtful use of that time to actually do something. Or learn a skill that you've always wanted to try, even if you're not thinking seriously about it as a career. It will still be good for refreshing your outlook on learning--and, one hopes, fun. Or if you just need to de-stress and clear your head, then find a low-demand part-time job (to keep some routine and intentionality in the week, and also for some resources to do the parts of de-stressing that require money), and use the rest of each week to re-connect with friends, build healthy habits, etc.
And then around November or December (when college apps start being due), you and DD agree to re-group and re-assess where things are. And again in the spring. And naturally, she can reconvene the discussion at any time, if she feels like she's already gained more clarity, or wants to bounce thoughts off you. If the discussion toward the latter end of the gap year is leaning toward not-college (at least at this time), then at least you'll be having a conversation with a young adult (or nearly so) with some more varied experiences in hand, rather than a middle teen trying to manage stress during a pandemic!
