When interviewed about the history of Peanuts for a biography on Schulz, both the man himself AND Joyce, along with many of their closest friends and collaborators (including noted lesbian tennis player Billie Jean King, whom Marcie was loosely based on) were interviewed. Peanuts, to them, was a retrospective look-through at Schulz's life, with him as the title blockhead and the Little Red-Haired Girl (called Heather in the TV specials, which Schulz considered non-canon, since the girl was never seen in the actual comic strips) being based on a co-worker whom Schulz blamed himself for not asking out, even when, per an interview with the co-worker in question, she herself liked "Sparky" but knew that the man wasn't much of a conversationalist and often put himself down beneath others, so it wasn't apparently meant to be.
No problem. When he and Joyce had their divorce, it right around the time that Lucy stopped actually liking Charlie Brown as a certain piano-playing prodigy who idolized Beethoven was introduced to the strip.
Well, "Sparky"--with the brand as a self-reflection of his life through adults in kid bodies basically, with Charlie Brown being his alter-ego essentially--based Lucy on the negative traits of first wife, Joyce Schulz.