Zeraora being electric/fighting is the same as people arguing about electric/dark luxray: there's about 1 to 2 design choices that could argue about it, which is very little to go by, but everyone talks as it is obvious
Zeraora is bipedal and has big hands. That's about it. Plasma fists is barely a fist move either.
Luxray has dark fur and a mean face. That's also about it
Aside from the arguments Pumpkin made about the moves it learns (which, yes, it's not the first Pokemon to lean into another type's flavour without being of that type, but still) for me personally the thing with Zeraora isn't so much that it feels like it should be, but that the fact that it isn't is such a missed opportunity.
Speedy frail Electric Pokemon are not a new thing, but there are very few speedy frail Fighters, and Electric/Fighting hasn't been done before. There are very few Fighting legendaries, and while maybe they thought that two Fighting mythicals would be too much for Gen VII it still strikes me as a waste that they didn't give it a second type. Zeraora is perfectly fine as a sole Electric mon, but it would be buckets more interesting if it were part-Fighting.
Luxray I don't personally have much of a stake in, but I will say I find it odd that apparently so many people think that a Pokemon specifically called out as being brave and heroic should be Dark.