Yes, phone cases can overheat phones. Phones are designed to vent themselves and cases can get in their way. In most situations, your case probably isn’t going to cause a major problem. However, when exposed to direct sunlight, kept in your pocket during a particularly hot day, or used for higher-processing tasks like gaming, your phone might have problems cooling in a case. If it does, the solution is to simply remove the case for a bit and let your phone cool.