In a novel I am reading a character says,
I could fix you some herbal tisane.
Is this redundant? (And to be picky, does one 'fix' tea?)
When you write your novel, you can be more succinct. I suspect this author slightly misunderstood tisane, or was concerned the reader might not understand it.
Hard to be sure without some context, but if I were going to nitpick (and I do like to nitpick too), I might be annoyed about tisane and fix not matching stylistically very well. I suppose you might be right that herbal tisane is rather like potato latkes. But herbal tisane doesn't bother me as much as it bothers you, and it doesn't bother me as much as the latkes example. We are in a pretty subjective realm with these criticisms.
I think fix may be more common in the South of the United States than in some other places.
It looks like a mashup of herbal tea and tisane. So, strictly speaking, it is redundant.
Herbal tea, or, more properly, tisane (UK /tɪˈzɑːn/, US /tɪˈzæn/), is any beverage made from the infusion or decoction of herbs, spices, or other plant material in hot water, and usually does not contain caffeine. Wikipedia