Can I over charge my mobile phone or tablet?

This is a common misunderstanding with electronics. The total power capacity (wattage, W) or current capacity (amperage, A) cannot damage electronics. The only thing that matters is the voltage provided (5V), which is standard for all USB interfaces. Chargers can simply provide more power (current) if the device draws it, but cannot harm the device because it provides strictly 5V.
The charging function of the battery is managed by the phone/tablet/device. All modern devices stop charging the battery when full, it has nothing to do with the charger, but instead is a function of the circuitry inside the phone or tablet. If you have a device that doesn’t manage charge cycles, you would know it (they are not typical consumer electronics).