Yes, U Can.

SQL> create materialized view testv as select sysdate onecol, sysdate twocol from dual;

Materialized view created.

SQL> desc testv
Name Null? Type
----------------------------------------- -------- ----------------------------
ONECOL DATE
TWOCOL DATE

SQL> alter table testv drop column twocol;

Table altered.

SQL> desc testv
Name Null? Type
----------------------------------------- -------- ----------------------------
ONECOL DATE