Burnett does receive credit, especially within the comedy community, but yes it does seem less in comparison to many male counterparts. Given the time period she made it big, and the rarity of famous female comics at the time, her contribution was actually greater socially than just her comedy, and that role is the role she likely receives less credit for.
Not only does Carol Burnette receive less credit, but she also had a harder time getting into comedy because she is a women. Even today there are still people's, mostly men who say that women have no place in comedy because they are not funny or that they are only funny to other women.
Carol Burnett is often held up as an icon of American comedy because of her work, regardless of gender, as well as because of the fact she succeeded in a environment that was heavily skewed in favor of men in the business. For both of those reasons she has received a great deal of credit. If anything, her gender allows her to provide a role model for all comedians, especially women in comedy.
Carol Burnett is one of the biggest names in comedy history, and that's almost certainly because she's female. There have been countless male comedians in the days since Burnett, but she's remembered because she was one of the female pioneers of comedy. If anything, Burnett receives MORE credit because she's a woman. If Burnett was a man, there's a good chance nobody would even know his name.