I would really like my NPCs in game to have AI. It is core gameplay actually, but I do not have time to implement it on my own. Can I just use asset for it? Is it in spirit of the jam?

I have no problem with that. Their are not really any rules. If someone just submitted someone else's game, sure, I would probably remove it, but if you are using an asset to make something happen, give NPC some logic, but are building other aspects of the game, that is fine.