MY HERO II, with Dicky Cheung Wai-kin, Ng Mang-tat and Lam Kin-ming. Directed by Chu Kai-sang. On Newport circuit. DICKY Cheung is a talented comedian/singer who, unjustly or not, is often described as a second-string Chiau Sing-chi. If Cheung hopes to free himself from that shadow, he should avoid sequels in which he inherits a role originated by the king of mo lei tau (nonsense) comedy. This is precisely the case with My Hero II. Cheung, like Chiau in the 1990 original, portrays a comic book illustrator who has difficulty separating fantasy from reality. While this may be a great premise on which to hang a screwball comedy, My Hero and My Hero II suffer from the same fatal flaw: they are unfunny. Chiau's presence allowed Part I to achieve modest box office success, but Part II, though not measurably inferior, has no such box office insurance. Part II is superior in that the subplots glorifying drug dealing and alcoholism are gone. Unfortunately, the void is not filled by anything diverting. A great deal of time is spent spoofing the gangster genre, but crime pictures have been the subject of dozens of spoofs in recent years. The scriptwriters come up with so little of interest for Cheung to do the spotlight is frequently stolen by two other characters: his mother and his hero. Tat (Ng Mang-tat) is an inept sharpshooter who becomes the inspiration for the comic book Cheung creates. Tat falls in love with mother (Lam Kin-ming), a nightclub singer who appears to be certifiably insane. Whether stealing everything at the supermarket or rolling around the floor as she performs a song, she seems a more likely candidate than her son for central figure in a crazy farce. While My Hero II is too fatuous to have a climax, a peak of sorts is achieved on the occasion of the first dinner date between Lam and Tat. They are both dressed 1960s style, she with her tiara-ed beehive and he sporting an Elvis toupee. With a little extra creativity, their Spanish dance might have been a comedy classic instead of another silly moment. The film is proficient, with the usual quota of martial arts and pyrotechnics. But the spirit is inadvertently summed up by Cheung's publisher when he examines the author's comic book and exclaims: ''The content is a bit empty, though the art work is good.''