In clamper circuit why RC>>T ? and why diode becomes reversed biased as capacitor becomes fully charged?
The diode here provides a path to the ground for the current to flow initially as the capacitor charges to the peak value of Vp.Once charged the right plate is at a negative voltage, so the anode of the diode faces a negative voltage and since it's obviously below its cut in value,it turns off. (even if the input is still at Vp).

Now, once the input goes negative, the left plate of capacitor is now facing negative voltage, so consequently the right plate tries to develop positive voltage and luckily the resistor provides a path to the ground.The result is that right plate develops a positive potential by dumping electrons into the ground through the resistor and thus starts to discharge.

The whole point of a clamper(The circuit is a NEGATIVE VOLTAGE CLAMPER) is to shift the DC level which is actually the task of the capacitor which should maintain its voltage to act more like a BATTERY.
The resistor here ruins it as the capacitor discharges.

A large RC (time constant) means the longer the capacitor takes to discharge and that's what we want, we want the capacitor to discharge  as slow as possible.