Can I recharacterize a Roth conversion if my investments dropped right after I did it?

No. The Tax Cuts and Jobs Act permanently eliminated the ability to undo, or recharacterize, a Roth conversion. Before 2018 you could reverse a conversion if the market fell, but now a conversion is final once done, and you owe tax on the value converted regardless of what happens next. You can still recharacterize a regular contribution, for example switching a Roth IRA contribution to a Traditional one, before the tax deadline, but not a conversion. Because conversions are irreversible, size them carefully and consider doing them when balances are temporarily depressed, so you convert more shares at a lower taxable value. Confirm the current rules at irs.gov before converting.