I was driving and my trucks thermostat suddenly jumped to super hot, took it to the mechanic the next day. Paid 250 for him to swap the thermostat and essentially treat it as an overheating issue. Pulled out of the lot and the thermostat jumped up again. Brought it back, he swapped the thermostat again for free and sent me on my way. Next time I took the truck out the thermostat jumped up and white smoke poured out of the hood. Had it towed to the same place and they said the head gasket was cracked.

Also, just replaced the tranny about 15k miles ago and had no leaks. Oil had some gold tint left and the mechanic put some more coolant in during the first visit.

Should I be asking questions or is this something that was not preventable or diagnosable earlier?

Sorry, I don't see how you might prove the head didn't crack first time it got hot. These 3.0's are notorious for blowing head gaskets and cracking heads.