The unusual public questioning of President Carter's leadership that was voiced here yesterday by the foreign policy expert of the leading opposition party drew sharp and varied reactions in West Germany today.

In a front-page editorial, the major newspaper in Cologne described the remarks by Christian Democrat Alois Mertes as "the most astonishing thing to come out of the opposition foreign and security policy for quite a long time."

Mertes, in an interview with the Frankfurter Allgemeine newspaper, said it was time to sound the alarm bell because Carter's policy and personnel shifts were undermining European trust in U.S. leadership to a point where it could present a mortal danger to the North Atlantic alliance.

He said it was especially crucial that such doubts be aired publicly because the "obvious weakness" in Washington was becoming apparent at the same time that the Soviets were skillfully courting the West Europeans in an effort to weaken their links to Washington.

The Cologne newspaper reported that while Mertes' views are his own rather than an official party statement, it is said that many leading members of the conservative opposition parties share his view and there are signs that Chancellor Helmut Schmidt privately does as well.

However, the foreign policy specialist of Schmidt's ruling Social Democratic Party, Marie Schlei, today sharply rejected Mertes' assessments and rebuked him for voicing them.

Schlei said her party "strongly rejected" Mertes' views about the supposed weakness of American leadership. She said such views were "irresponsible" and that such unqualified doubts about United States as Mertes expressed were the very things that could destroy existing trust.