Do I need to apply for a 30-day extension before applying for a Non-O 90-day visa in Thailand?

You don't need to do extension for 30days if you have more than 15days on your current stamp while applying for your 90days nonO but you do need the tm30 while applying

The only reason you would need to apply for a 30-day extension first, is if you did not have enough time on your existing stamp to apply for a non-O visa. You need between 14-21 days left on your current stamp in order to apply for the non-O conversion in Thailand, with the amount of time differing depending on the immigration office you are using.