It is never explained and Schultz left it to speculation.
Charles Schultz wrote The Peanuts as a work of children's literature. Marcie and Peppermint Patty are essentially (perpetually) ten years old. There is no evidence, hard or implied, that there is any sort of relationship between Marcie and Patty other than friendship. Short answer, no.
Peppermint Patty's best friend's name is Marcie. They are often seen together in the Peanuts comic strip created by Charles Schulz.
Patricia "Peppermint Patty" Reichardt and sometimes Marcie .
Patricia ["Peppermint Patty"] Reichardt
Peppermint Patty's closest friend is Marcie.
Charles M. Schulz the creator of the "Peanuts" comic strip .
Peppermint Patty's first guest was Snoopy in the Peanuts comic strip. She invited him over to her house for Thanksgiving dinner, but he ended up cooking a large meal in his own unique way.
Sir.
Peppermint Patty invites herself, Marcie, and Franklin.
Because it just silly enough to be funny.
Linus Lucy Schroeder Sherman Frieda Pig Pen Violet Peppermint Patty Marcie Franklin Rerun
According to Charles Schulz, no she was not. It is fairly obvious that she has a crush on Charlie Brown, whom she called "Chuck." The female whom she is often paired with, Marcie, also has a crush on Charlie Brown.