Although films like ''El Crimen del Padre Amaro'' and ''Y Tu Mama Tambien'' scored at the U.S. box office and the Oscars, Mexican president Vicente Fox is threatening to cut funding of such organizations as the national film institute. ''It's gonna hurt the Mexican film industry badly,'' says Guillermo Arriaga, writer of ''Amores Perros'' and ''21 Grams.'' The plan is especially hard on young filmmakers, who depend on state-funded shorts to make their mark. ''I couldn't have done 'Y Tu Mama Tambien' [without] my first film, 'Love in the Time of Hysteria,''' says director Alfonso Cuaron. The good news? Many chalk the proposal up to political posturing. Says Arriaga: ''Fox just needs to understand the importance of culture.''