What, MAD magazine worry? The answer in this case may be yes. Don Martin--for 30 years MAD's maddest artist--has abandoned the mother ship for Cracked, the competition. The reason: at MAD Martin didn't own his material, but at Cracked he will. That may make up for the audience Martin's losing with the move. MAD's circulation averages 1 million readers; Cracked's circulation averages 250,000.