This definition would make eating pizza itself a niche thing, topping or otherwise. 33% of the population is mainstream and it isn't an option to claim otherwise. Less popular? Yes. Fringe? No. Fringe is something like Linux on the desktop at sub-1% of the market. It is not feasible to ignore or marginalise 1 person in 3.
I'm certainly impressed by the position that the most popular show is the fringe one. In theory it is possible, but practically that is a hard sell.
So do you want to cut your customers who like pineapple pizza or your customers who don't like pineapple pizza?
Or... Maybe neither and sell to everyone?
He was one of the most popular cable, non-sports shows (his show and The Five, also on Fox News, swapped to the top spot back and forth recently), but cable-specific shows other than sports are (individually) niche.
If it turns out that Carlson's popularity is so tenuous that the truth of the antecedent in the argument is doubtful, that's great. But the argument is completely bogus regardless.
Or to put it more boring, mathematical terms, the mode is not the same as the median. Even if somehow one in three people were huge fans of Carlson, that would still count as relatively niche - and strongly controversial - because that would be the 1/3 most hard right of the US population.