Can a loan officer/ underwriter demand an ex-spouse’s bank statements?

Yes, they can demand this. You have to remember, there is no right to or guaranty of getting a loan, and no requirement that any given lender make any given or particular loan. Lenders are free to turn down loans if they feel they are risky, and they are entitled to determine for themselves what information they require to fully evaluate the risk of a loan. So this lender can decide that it needs your ex's financials to decide whether to lend to you, and can refuse to lend if they don't receive it. While you are free to contact other lenders and see if any will lend to you without this information, you cannot force this lender to forego or give up on the information it clearly wants.
It is reasonable for the lender to want this information, by the way. It is YOUR car payment: you are legally responsible for it, and if your ex does not pay it for you, you will have to pay it--increasing your monthly expenses and possibly making it difficult or impossible to also pay your mortgage. While your ex was ordered to pay, that court order counts for nothing if he doesn't have the money to pay--court orders do not make money appear where there is none. Therefore, it is reasonable for the lender to make sure that you ex actually has the financial resources to pay, so that he does not stop paying forcing you to pay.