ARE Americans eager for a ''flat tax'' they can fill out on post cards? Not quite, according to a new Associated Press poll. Eight in 10 Americans think the federal income tax needs some changes, but fewer than 3 in 10 favor replacing it entirely. So maybe it's not bad that Congress's tax-code changes this year amount to tinkering. Here are the major changes:
* If you report unemployment-compensation income, you may use Form 1040EZ.
* The Internal Revenue Service (IRS) now requires Social Security numbers for children born before November 1995.
* A health-insurance deduction for the self-employed has been restored. It expired at the end of 1993 and was restored retroactively to the beginning of 1994. If you are eligible to take this deduction on your 1994 tax forms (filed in 1995), you can file an amended return on Form 1040X.
* Military personnel on extended active duty abroad are now allowed to qualify for the earned-income tax credit.
* The rate for deducting auto-mileage costs, such as for charitable purposes, has gone up from 29 cents to 30 cents a mile.
* Finally, the IRS has expanded its ''ombudsman'' program to intervene on behalf of taxpayers in the event of a tax problem with the agency. The ombudsman can be reached through your local IRS office.