There are a few potential reasons why the iPhone 13 screen may appear dim. One possibility is that the brightness setting on the phone is turned down. Another possibility is that the phone's battery is running low and needs to be recharged. Finally, it is also possible that the phone's screen is simply dirty and needs to be cleaned.
If the brightness setting on the iPhone 13 is turned down, the first step is to check the phone's settings to see if the brightness can be increased. If the battery is running low, the phone can be plugged into a power source to recharge. If the screen is dirty, it can be cleaned with a soft, dry cloth.