Radovid's anti-mage policy is not a problem to his characterization. It is evident from TW2 that this is the sort of policy he would prefer to pursue, to get rid of them once and for all (he smirks at what happens in Loc Muinne, should a mage pogrom happen). It is a populist move, designed to appease the population and the Order of the Flaming rose (which apparently doesn't exist anymore), and to satisfy his own personal hatred.
That's fine, characterization wise.
What is not fine, is turning a cold calculating politician into a raving lunatic, who speaks and acts as such. In such a way, that even fucking Loredo looks saner and a lot more subtle. The whole chess scene was the worst scene I have seen of the game. It was just ridiculous, nonsensical, and just stupid.
It would have been a lot more terrifying to see someone who is sane, and yet still pursues such policies. It would have been a more interesting observation to the things humans are capable of doing in cold blood, for reasons of state.
The 2nd problem with Radovid is that there is absolutely nothing redeeming about him. His victory outro is pretty much saying "well it would have been completely better in every way possible to let Nilfgaard win." And him uniting the North could have been the only positive outcome, except Djistkra accomplishes the same, and does it better.
So the game is pretty much telling you that Radovid winning (through your inaction btw, and not an active choice) is the worst possible outcome. The bad ending.
I would have much preferred a nuanced ending, with no ending being clearly superior to the others (in this game, there IS a good happy ending, and that's Nilfgaard winning). Instead, each ending has its pros and cons, and people would weight them in and make their choice.
EDIT: I would have also liked being able to call Djiskra on his BS and remind him that he was the one who funded the Order of the Flaming rose and help them expand in the North.