You could definitely argue that it was, but there is no one correct answer as to what the main factor in ending the war was.
The war ended because Richard Nixon felt that the US had to get out of the war. He saw how unpopular the war was and decided the war needed to end. He promised "peace with honor" and got the US out of Vietnam in 1973, five years after the Tet Offensive.
Nixon won the election of 1968 in part because Lyndon Johnson dropped out of the race. Johnson dropped out largely because of public outcry over the war after the Tet Offensive. The public had been led to believe the war was practically over and then Tet showed that was not true.
In this way, you can say Tet led to Johnson dropping out, which led to Nixon winning. Nixon's win led to US withdrawal. By saying this, you can argue that Tet was the main factor.