DD is rushing (virtually) at her university. I wasn't in a sorority and have been somewhat skeptical of the value (and worried about the downside). But now I'm worried that she won't get into her top picks or will be rejected altogether. Crazy right?

Mostly looks, I would say, but also personality. Each sorority has its own look and feel, and for the most part, they’re looking for people to fit the mold. They use the grades and scores to get rid of people, in my opinion - they don’t want academic liabilities because then they’re put on probation (and their social activities are limited).
I would say this rings true for the “popular” or “most desirable” houses - the ones woman want (and can be the most women clamoring to join) and with which the men all want to hang out. As you go down in “tier” (or whatever system people use for ranking them), the emphasis on looks and personality lessens. For the sororities at “the bottom”, many will take anyone who wants them.
I say all of this all as a former member of a sorority (who would encourage women to find outside interests) - looking back, I am glad I kept friendships even when they weren’t deemed important or valuable by the Greek social structure (even though it made things difficult for me, in some respects, at the time - how screwed up is that?!). It’s so easy to get caught up in the exclusivity of it all. No matter what happens, please affirm (and reaffirm) that no matter where she lands, her self worth and value are not determined by her house, or its placement in the larger Greek social structure - that can get lost in the process.
Bonus: almost no one cares about this stuff after college.
Wow. Not at all the metrics in the 90's. Mine I felt was just did you have adequate social graces, were more or less reasonably attractive and outgoing (enough). That's all!
I was a sorority woman at a small college in the northeast. Something like 40% were greek, but it wasn't a big deal if you weren't. The whole thing was pretty laid back when compared to the big southern schools. I get your apprehensions, though. I was not a typical sorority-type and I went into with with an open mind and the idea that I could always drop out at any time. I'm glad I did it. I made great friends!
OP. She is attending a midsized school in the Bay Area. Greek life is not a huge thing and they only have a handful of sororities. I don’t know all the details about virtual rush, but it seems like a difficult venue in which to shine and show your personality. I guess I should be relieved that she’s not in a super competitive school in the South. But I know she’ll be disappointed if she gets rejected. I’ll ask her if she needs letters of recommendation. Looking at reddit is also a good idea.
I was in a sorority in a texas university. It was low stakes rush though... not at UT or anything. She needs to go in with the mindset of seeing if it’s for her. It’s really not a big deal. She can even pledge and decided not to get fully initiated. I ended up doing an art major and hanging out with the people in my major over the sorority during my junior/senior years. There were some good times but it wasn’t the end all/be all of college for me.
If she is rushing an established sorority you might be able to help her out by getting recommendations ("recs") from alums of that sorority. Many many non-Greek moms miss this opportunity and - it can make the difference for young women who are on the cusp of getting in.
May I ask what school? I was in a sorority at a large southern university. There is value I guess, but in retrospect, it's not the end of the world not being one either. How are they going about it virtually?