Some Democrats in Congress applauded President Reagan's nuclear arms reduction proposal yesterday as a good first step, but most early reaction was skeptical, emphasizing that his plan still would not halt the arms race because the United States and the Soviet Union could continue modernizing their arsenals.

Former secretary of state Edmund S. Muskie, making what was billed as the formal Democratic response, said he feared Reagan's proposal for a one-third reduction in nuclear arsenals might be "a secret agenda for sidetracking disarmament" while America rearms.

Sen. Edward M. Kennedy (D-Mass.), a leader of the nuclear freeze movement, said, "Behind the rhetoric, the reality is that President Reagan's proposal would permit the United States to build the MX missile, the B1 bomber and an entire new generation of nuclear weapons.

"This is not what the American people mean when they call for arms control," he added.

Sen. Sam Nunn (D-Ga.), a member of the Senate Armed Services Committee, said he thought the president's plan was "a good start." But he said the negotiations would be lengthy and suggested that Reagan "seriously consider taking SALT II and then proposing any amendments he thinks necessary." (SALT II refers to an unratified treaty arising from strategic arms limitation talks.)

Sen. Henry M. Jackson (D-Wash.) said, "I applaud the president's proposal . . . . This is a beginning." He noted that two previous arms control treaties had resulted in nuclear buildups and said the goal must be "elimination of all nuclear weapons."

Rep. Thomas J. Downey (D-N.Y.) called Reagan's plan "a step in the right direction" but added that to be effective it also would have to curtail "destabilizing first-strike weapons on both sides." In the meantime, he said, "it would be prudent for us to have the Soviets bound by the SALT II limits."

Meanwhile, there were Mother's Day marches against nuclear weapons over the weekend in Boston, Minneapolis and Terre Haute, Ind. About 1,500 people turned out in Boston Common. "Don't Nuke Me" read one sign on a baby carriage containing a sleeping child.

Muskie said giving "formal approval in some way" to SALT II is the way "to freeze the balance at rough parity." Failure to do so will result in a renewed arms race on both sides, he said.

Recent statements by White House counselor Edwin Meese III seemed to indicate the United States wouldn't abide by the terms of the unratified SALT II treaty, he said, adding, "I am afraid that what this may mean is that we are declaring a 'free for all' for several years in the arms race."

Retired Rear Adm. Gene LaRoque, director of the Center for Defense Information, said the president's speech was evidence he "is gradually learning" about the surpluses of nuclear weapons on both sides. But he said the plan "will do little to slow the arms race as long as his proposal stands for the construction of 17,000 nuclear weapons in the next 10 years" for the MX, B1 cruise missiles and others.

Randall Kehler, national coordinator for the Nuclear Weapons Freeze Campaign in St. Louis, said: "Either the president is not listening to the American people or he is ignoring their message. That message is: Halt the nuclear arms race!" He, too, noted that the proposal would permit both sides to go forward with a new round of weapons.