LEXINGTON, Kentucky: A surprisingly cheerful Jim McDougal reported for what he called a three-year "adventure" in jail, dropping a word of warning for President Clinton. Asked by reporters outside the Federal Medical Center Institution in Lexington if his cooperation with Whitewater prosecutors would prove damaging to the Clintons, McDougal allowed that it certainly couldn't help them. Maybe so, but McDougal has little credibility. His willingness to cooperate with Starr has slashed his possible 84-year prison term to just three years for his role in the collapse of Madison Guaranty Savings and Loan. The real qustion that must be worrying the Clintons is whether he is backing up his stories with evidence of any wrongdoing. McDougal may be colorful, but isn't colorful financing the charge behind Whitewater?