"Important" as in "significant to the main character", not story-wise. And it does make sense to corrupt those two. After all, that's what "corruption" is all about - taking something pure and defiling it into the opposite.
The opposite of Tish is someone who doesn't like to put her dick in anything that moves, so that would not work out well for anyone.
As for the mothers, meh, I don't like incest and even if I did, they are already as dirty as they come and have the bad habit of losing their children, so not much else to do in the corruption department. Not to mention how much of waste it is to spend all the resources needed for a proper corruption in characters that I would not even call secondary.
That said, if anyone needs a proper corruption route it's Clawyn, she has very little changing if you corrupt her.
And Zent, and by corruption, I mean turning him into Zenta permanently (or not, I don't judge).