Why does the bias decreases in this clamper circuit?
During the negative cycles, the capacitor needs to charge so that a positive potential develops on its right side plate. This voltage will add to the waveform during rest of the operation of the circuit.
To charge the capacitor during negative half cycles only, the diode needs to be conducting during negative half cycles. -100mV amplitude is not enough to forward bias the diode and make it conducting.
Repeat your experiment with say 2V amplitude triangle wave and see if the situation improves.
To work with low voltages, you need either

A precision clamper circuit
A diode with a much smaller forward voltage drop.
A voltage source which biases the diode to the threshold of conduction (or larger value ). Indicated as \$V_{ref}\$ in the linked Wikipedia page.




simulate this circuit &ndash; Schematic created using CircuitLab