Viola has several things stacked against it: no one knows how to pronounce it, it's a weird instrument, and "Violet" is right there and way easier to deal with.
I think it would have shed its old-lady association more if it hadn't had these things working against it when the trend it fits into was picking up.
"Throckmorton" is just a cumbersome sounding name, that's why it's funny. Maybe Viola is a prissy name? So Isabella Throckmorton?