I might be missing something, but it doesn't seem like there is a way to customize the name of an association in the form builder.
I would prefer it to even use the actual association names but right now it is using the table name.
In many cases I have different associations for the same table so there ends up being multiple of the same optgroup labels.
You can use i18n for that. See https://github.com/ernie/ransack/blob/master/lib/ransack/translate.rb