The unobtrusive monitoring of vital parameters has recently gained significant importance in home care and the automotive industry to supervise the driver's awareness. In wearable devices, contact-based photoplethysmography (PPG) enables the estimation of heart rate, respiratory rate, blood pressure, and peripheral arterial oxygen saturation through the optical acquisition of blood volume variations in the skin. This work analyzes the feasibility of unobtrusive acquisition of the PPG signal through clothing using Monte Carlo simulations. For that, we suggest a male and a female model of the human back skin covered with six interchangeable clothing items with different tissue, color, and thickness. The arterial pulsation is simulated through blood volume variation in a modeled vessel during systole, diastole, and end-diastole. The absorption coefficients of each clothing were measured with an optical power meter and modelled accordingly. Nine increasing distances of 0 to 2.95 cm between a red (760 nm) or infra-red (900 nm) Light Emitting Diode (LED) and a photodiode (PD) are investigated. The quality of the PPG signal is evaluated by considering the detected light power, the order of the detected light intensities at systole, diastole and end-diastole, and the perfusion index. The simulations revealed that the ability to detect the PPG pulse through clothing depends on the thickness and type of fabric used, the LED-PD distance of the sensor, and its wavelength. The optimal LED-PD distance was found to be 1.2 to 1.8 cm. The infrared LED generally generates reproducible and better quality PPG signals for cotton and polyamide T-shirts, but not for a thick gray cotton hoodie and a thin acrylic red sweater. Besides, significant differences are reported between the male and the female models. Our results evidence the limitations of PPG monitoring through clothes and suppose a step forward in unobtrusive sensors' design for hospitals and smart cars.