What percentage of fish are herbivorous vs. carnivorous?
I don't have an answer to this but there are a number of elements I think can be helpful for you in finding an answer.

First, a helpful word for searching here if you didn't know or hadn't thought of it is "trophic", as in "trophic level", as in "how high this organism is in the food chain".

Making searches related to fish and trophic levels I didn't find a single result for all fish; there are plenty of papers and graphs about the trophic distribution of fish but it's usually within a specific context or ecosystem. For example this graph gotten from Google Images, from a paper that looks at reef fish in Hawaii:



You can see in that graph, and plenty of other papers confirmed, that "herbivore/carnivore" may not always the most relevant distinction in terms of what a fish eats; unlike on land, where the base of the food chain is plants which are pretty big and visible and so are most of the organisms that eat them, in the ocean the base of the food chain is phytoplankton, which is eaten by zooplankton, both of which are what most of the fish we consider "at the bottom" of the fish food chain eat, so by that standard almost all fish are carnivores. But it doesn't seem to be how the word is used by everyone in the field; the graph in question being an example, distinguishing "Benthic carnivores" from "Planktivores" (Plankton-eaters) and "Piscivores" (fish eaters; presumably the "carnivores" eat non-fish animals).

See also this graph looking at reef fishes at various sites in the Caribbean:


Or this graph looking at the trophic structure in a stream


both of which similarly distinguish things more finely than "herbivore/carnivore".

And this, that I got from a search for "fish trophic histogram", ties the concepts to trophic levels (but on a local scale only again, this time the Mediterranean):


Otherwise the FishBase database seems to contain trophic pyramids but they need to be seen per ecosystem and there are a lot of available ecosystems, and given the shape of most pyramids they might include more than just fish.

As an aside, this paper might give a good explanation for why most of the fish we eat are carnivorous:
Trophic level scales positively with body size in fishes