What should I do if I was mistakenly given a 30-day entry instead of a 60-day tourist visa in Thailand?

I had this problem a couple of weeks ago, I went to immigration and told them what happened and they made me fill out a short form and then swapped my 30 day exemption for the correct 60 day visa, though this was within the first 30 days so I didn't overstay, I think it should still be a similar result since you likely added your visa number to your arrival card so you did the right thing.

That's not a problem

If it is a tourist visa, go to immigration asap, they'll correct it

Airport only give 30 days on arrival, it's not a tourist visa it's a visa exempt. For 60 days, you need to go to Laos, Malaysia, Vietnam, etc. If this is how you got to the country (without doing a proper visa), u will be charge 500b per day so about 11k if you go extend money.

If you did ur tourist visa in another country prior to entering by air, it's a mistake on their part and if u go to extend and tell them you won't have to pay any fine.