Yes
**EDIT**
No she wasn't. Charles Schulz himself said she wasn't, and considering he's the creator of her, it's safe to assume he's the correct one.
Further evidence is the obvious crush both she and Marcie, one of the other characters thought to be gay, have on Charlie Brown.
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.
Patricia "Peppermint Patty" Reichardt and sometimes Marcie .
Charlie Brown
Gini Holtzman was the voice actress for the Charlie Brown comic series character: Peppermint Patty from 1984-1986.
peppermint patty
The two girls who dance in "A Charlie Brown Christmas" are Peppermint Patty and Sally Brown.
Charlie Brown is the character who has a friend named Peppermint Patty, as well as friends Linus and Snoopy, in the "Peanuts" comic strip created by Charles M. Schulz.
Linus van Pelt was Charlie Brown's friend who used to call him Chuck. He is known for his security blanket and philosophical observations.
Charlie Brown, a character created by Charles Schulz, is depicted as a heterosexual boy. In fact, most of the Peanuts characters are depicted as heterosexual, with the possible exceptions of Peppermint Patty and Marcy, though this is a matter of pure speculation.
In the Peanuts comic strip, there are characters who show affection for Charlie Brown, such as Peppermint Patty and the Little Red-Haired Girl. However, the nature of these feelings varies and may not necessarily be described as romantic love.
Patricia ["Peppermint Patty"] Reichardt
The Charlie Brown and Snoopy Show - 1983 Peppermint Patty's School Days 2-4 is rated/received certificates of: Australia:G