Is magnetotherapy effective at treating dental problems?

TMJ. There may be some utility wiith TMJ inflammed conditions, though I am not aware of any clinical trials.
See your dentist. Magnet therapy is used today as a treatment for pain and inflammation. However, evidence that magnet therapy helps in any of the TMJ, tooth pain or gum disease is still highly preliminary. And still needs to be validated in properly designed (RCT) trials. See your dentist for evaluation and traditional treatment.
No evidence. There are currently no evidence or studies showing magnetotherapy to work against dental problems.