In the TH rules, it states this about art ownership:

"Please ensure all artwork you use is used with permission and properly sourced."

Since a large majority of AI art is created through machine learning (using countless numbers of art without consent), it should be strictly disallowed through that rule
Though, I think due to there not being many people moderating or updating Toyhouse, the rules haven't been updated to account for the recent major uptick in AI art right now