Is there a word to describe an appearance between pretty and ugly ?
"If you call a woman "plain", you're saying that she's not pretty - though you're not saying that she's ugly."
That being said, I could imagine a context in which "plainness" would be seen as a positive quality - it could be seen as positive through comparison with people who care too much about their appearance. I could also see something like "She was a plain farmer's wife, with the same blank beauty as the prairies that surrounded her."
The term, "blank beauty" is an interesting turn of phrase, to me.
ADJECTIVE Not beautiful, distinctive, or remarkable in appearance.
lacks distinctive beauty
of ordinary attractiveness