Is it acceptable to have a return ticket that is more than 30 days after arriving in Thailand with a 30-day visa exemption?

The airline will want to see onward travel within 30 days. The 30 days extention, although rarely refused, isnt a given and the airlines know this.

First of all, it's visa exempt not visa on arrival. Visa on arrival is only for 15 days where you BUY a visa when you arrive, and only available to a few countries.

Second, the airline will generally want to see proof of onward travel (not necessarily return travel) within 30 days since that's what you'll be entering with. You can buy a cheap throwaway bus ticket to a neighboring country, or a cheap throwaway flight to a neighboring country.

You can also google "Onward travel ticket" to see what other options are available.