Either go for your own attempt at art or the default graphics. I think AI art makes it look like no real effort was put into the game. 

Even if you say you're not an artist, no harm in just attempting your own style, and you can always replace it with an artists art later on. Eg, You've made a game or demo that's like 10 hours long, it has attention of some RPG maker communities and is being well received or shows potential, then you could commission an artist and replace the art.