Can we marry in the US with her US citizenship now?

You are already married and cannot marry again without first getting a divorce in Mexico. Also, there is no reason for the second marriage because the U.S. recognizes marriages registered in other countries under 2 conditions: 1) the marriage must be valid under the laws of the country where it was registered; and 2) the marriage must not be against the laws of the United States (for example, the U.S. will not recognize the marriage where the bride is 12 years old or a marriage where there are more than 2 spouses) There should be no problem with recognition of your Mexican marriage in the U.S. As to your wife's having a dual citizenship, it is not uncommon and does not violate the law.