I assume you mean protection against the power supply being reversed. Yes, the diode will drop around 0.7V. This probably doesn't matter unless you are using the supply voltage as the ADC reference.
On their motor drivers, Pololu uses a mosfet to isolate the chips if the motor supply is connected the wrong way round. You could do the same thing using a small P-channel mosfet to switch the positive supply of the ATtiny. Note that the source and drain need to be connected the opposite way round from usual (i.e. source to ATtiny's Vcc pin, drain to the incoming positive supply), due to the presence of the body diode.
Either way, you will need resistors in series with any connections from the Dsub to the inputs and outputs of the ATtiny as well, to limit the current flowing into the internal pin protection diodes when the polarity is reversed.