"Garry Trudeau put me in the Doonesbury strip many years ago. So I've been a cartoon once"

Steve Cropper is referring to the fact that he was featured in the Doonesbury comic strip by Garry Trudeau. This is a considerable accomplishment for Cropper, as it is a testament to his popularity and impact in the music market. The quote implies that Cropper was featured in the cartoon many years back, which recommends that he has been a popular figure in the music market for a long period of time. It also suggests that Cropper has an excellent relationship with Garry Trudeau, as the cartoon creator chose to feature him in the strip. The quote is a humorous method for Cropper to reveal his pride in being included in the Doonesbury comic strip, and it is a pointer of his long-standing influence in the music market.