It seems very unlikely that you would have to file in Mississippi, unless you had Mississippi withholding on either your SS benefits or your pension.
To quote the State of Mississippi:
If you are a Mississippi resident or have Mississippi income and the income exceeds the allowable deductions and exemptions, you are required to file a return. If you are not required to file a Mississippi return, but you received a W-2 form stating you had Mississippi tax withheld, you must file a Mississippi return to get a refund of your Mississippi withholding.
And what are the income limits?
• Single resident taxpayers – you have gross income in excess of $8,300 plus $1,500 for each dependent.
• Married resident taxpayers – you and your spouse have gross income in excess of $16,600 plus $1,500 for each dependent.
Also note that Social Security benefits are not taxable in MS, as well as "normal" retirement income (i.e., not an early distribution). "Social Security benefits, Railroad Retirement benefits, and retirement income from federal, state, and private retirement systems are exempt in total." (page 12)