Kingmakers controller support
Unknown. The Steam page marks no controller support of any kind, and the developers have not announced any. For a game that is not out, a blank Steam field means nobody has declared support yet; it does not mean support has been ruled out.
No controller support is marked on Steam, and none has been announced. For an unreleased game that is a blank, not a refusal. What would change this: the controller fields filled in on the Steam page, or a developer post about pads.

What the Steam page records
Steam stores controller information as a set of yes-or-no fields that developers fill in. For Kingmakers every one of them is currently no:
The Epic Games Store page carries no controller information either. When a player opened a thread on the Steam forum about the missing controller label, no developer replied in it.
The only developer remark about game pads
It came up sideways. Explaining why the strategy interface was rebuilt, the game director said the old one did not suit pads:
“Also for game pads this system wasn’t going to work well.”
That shows controllers were a consideration when the interface was redesigned. It is not a statement that the game will support them, and it should not be read as one. The redesign itself is described on what Kingmakers Early Access includes.
Why it is a hard question for this game
Kingmakers is two games at once: a third-person shooter and a strategy layer where you place buildings and order troops from above, like the view pictured at the top of this page. That strategy layer is the part the director said was rebuilt partly because the old version did not suit pads. How the two views work is on Kingmakers gameplay explained.
If you play on Steam Deck
This matters most there. Steam Input can map keyboard and mouse controls to the Deck’s buttons and trackpads for any game, but that is not the same as the developer supporting a controller. The Deck answer is on Kingmakers on Steam Deck.