As long as you're getting pensions from the US and Social Security, or dividend or distribution income from US investments, you may be liable for US taxes and be required to file federal and state returns.
Dual citizenship in another country (e.g., Mexico) side-by-side with your US citizenship may have tax consequences in the second country.
If you have significant income other than Social Security in the US and are thinking of dual citizenship (for the right to vote, get into politics, not have to check in and out at the land borders) it wouldn't hurt to talk to a tax accountant in the US who does international taxes.