Distributions from a tax-deferred retirement account, including distributions for Roth conversion, are taxed as ordinary income -- it's not a capital gain. Since you're in the 12% federal bracket, that's the rate at which you would pay tax for your proposed conversion. The strategy you're probably thinking about, tax gain harvesting, is if you're in the 0% LTCG bracket (like you are) you sell and repurchase in a taxable account to get a step-up in cost basis.
Edited to correct tax bracket to MFJ.