Asian-American community leaders in San Francisco are mounting a campaign to alter the proposed 1990 U.S. Census forms, which lump together more than 30 ethnic groups--including Chinese, Samoans and Filipinos--under the heading "Asian and Pacific islanders." Critics say the forms could skew the results of population studies that rely on census data and deprive many bilingual and affirmative-action programs of federal funds. Asian leaders are calling for a return of the 1980 census form, which listed many Asian groups separately, or the development of an expanded form to reflect even more ethnic groups. Census Bureau officials said the preliminary new forms were developed because of recent legislation requiring the bureau to provide a total count of Asians by Jan. 1, 1991. Without the new form, the numbers could take too long to compute, a bureau spokesman said.