New York Gov. Mario M. Cuomo ripped President Reagan's tax-simplification plan yesterday as an "assault" on the middle class, a "pork barrel for the very rich" and a "radical proposal" that would "undermine the philosophical underpinnings of our federalist form of government."

Cuomo's chief target in a speech at the National Press Club was Reagan's proposed elimination of the deduction for state and local taxes by taxpayers who itemize on their federal returns.

Cuomo, a Democrat, declared himself a staunch supporter of tax reform, which he said has been enacted in New York, but denounced Reagan's proposal as the opposite of reform. "This plan, I am sorry to say, is a rip-off dressed up as a reform," Cuomo said. "This administration would call it tax reform to slap a new tax on the unemployed while lowering taxes for the rich by 10 percent . . . . It is a retreat from fairness, an assault on the middle class and a pork barrel for the very rich."

Reagan, speaking in Indianpolis yesterday, defended his proposal as one that would lower tax rates for most Americans. Administration officials say lower rates would offset the loss of the state and local tax deduction in most cases.

Cuomo, however, accused Reagan of pitting the 15 "high tax" states, whose citizens benefit most from deducting state and local taxes, against the other 35.

He also denounced the president for using "new math" in advocating eliminating the deduction for state and local taxes rather than eliminating or narrowing tax loopholes for corporations. Cuomo said the administration's first tax-simplification proposal, prepared by the Treasury Department last November, would have decreased corporate depreciation write-offs by $68 billion by 1990, while its revised plan would decrease them by $15 billion.

"By retreating from its first plan, the government gave back $53 billion to corporations in depreciation allowances alone, and it gave back $31 billion to financial institutions over five years," he said. "Altogether, the administration scaled back 44 percent of the corporate reforms that the first plan undertook."

Cuomo said restoring some of these taxes and others such as foreign tax credits was a better way than tampering with local tax deductions to get the $40 billion the administration says is necessary to lower overall tax rates.

In a question-and-answer session after his speech, Cuomo slid away from the inevitable inquiry about whether he will seek the Democratic presidential nomination in 1988. "I'm getting ready to run for governor in 1986, and that's it," he responded.