Harvard defense observers yesterday gave mixed reviews to Reagan's recent proposal for deploying the MX missile, with several terming it politically motivated and militarily unsound.

"Of all the decisions he could have made, it is the silliest," Stanley Hoffman, Dillon Professor of the Civilization of France and a frequent administration critic, said.

Several Harvard observers, including Hoffman and Albert Carnesale, professor of Public Policy at the Kennedy School of Government, attributed political motivations to Reagan's disavowal of the Carter administration's "shell game" concept of scattering 200 missiles in 4600 new bases in several Western states.

That concept, favored by many in the Defense Department, drew criticism from conservative quarters in those states normally friendly to defense, such as the Morman Church in Utah and Republican senators like Paul Laxalt (R-Nev.).

The chief military disadvantage, several observers said, was that the MX basing system adopted by Reagan--placing 100 MX missiles in existing silos--would not reduce the long-term "vulnerability" of land-based U.S. missiles to Soviet attack, the reason the MX had been proposed in the first place.

A majority of more than 300 Harvard undergraduates surveyed yesterday expressed disagreement with President Reagan's decision this week to go forward with a scaled-down version of the MX missile.

Students questioned randomly in a door-to-door poll also opposed, by 56 per cent to 18 per cent, Reagan's approval of plans to build a fleet of B-1 bombers to replace aging B-52s.

Reagan's move, announced last week after months of speculation and study, calls for the placement of 100 advanced MX missiles in existing silos for Titan and Minuteman rockets. The plan replaces the so-called "shell game" proposal advocated by the Carter administration, in which 200 MX missiles would be shuttled among 4600 new missile shelters.