FOUND IT! (Which is to say: Daly’s archive didn’t lie about the early gendering of her fan letters):
“Daly recalls that in the first few years following publication, ‘All the mail was from boys, perhaps because of the attractive girl on the cover. Later, the girls started writing, too.'”
Thank you, now-obscure author interview from 1986!
To be clear, the archive suggests that girls *were* writing from the beginning, but I suspect that Daly remembered this version because the girls continued to write to her, whereas the boys stopped.
Source: Daly, Maureen, and Kimberly Olson Fakih. “The Long Wait for Maureen Daly.” Publishers Weekly 229.26 (June 27, 1986): 36-39.