Not directly, no. Anyone invested in the stock market will lose alongside them, too.

As for whether we "gain" anything indirectly from having less-wealthy wealthy people, that's arguable. And even on the bits that are arguable, it's going to depend on why their wealth decreased and what, if anything, the government does in response.

But again, we don't directly gain anything from having less-wealthy wealthy people. And the hold the wealthy have on political action—"influence," whatever—means the sort of policies that would allow the general population to benefit from their loss are mostly nonstarters.

In the ideal version of capitalist competition, their losses would spur them to redouble their efforts to make their companies competitive, lowering prices, raising quality, hiring workers to improve overall production... etc... But that lowers the dividends their stocks will pay out, and in our current reality, that's also a nonstarter. The goal is to raise the value of the stock, not to build a stable company. (See, e.g., comparisons between Tesla's market cap and the combined market cap of Ford, GM, Toyota, Honda, and the next five largest automakers.)