It doesn't hurt. The Apple 13 can be slowly charged with 5 watts.
The iPhone13 series supports the previous charger charging, but the charging speed will be slower, if you use a 5W charger is expected to take 2.5-3 hours to fully charge.
The compatibility list for the 5W power adapter includes the iPhone 13 series. 5w charger charging is more difficult to heat, will better protect the battery. In practice, the 5w and 20w chargers bring little damage to the battery, which is small enough to be negligible.