Is the Mach number shown by an Air Data Computer considered a "true" Mach number?
For subsonic aircraft the Mach number is normally measured from dynamic pressure and static pressure, very similar to the way calibrated airspeed is measured.

$$M_a=\sqrt{\frac{2}{\gamma-1}\left[\left(\frac{q_c}{p}+1\right)^\frac{\gamma-1}{\gamma}-1\right]} $$

with:


$M_a$, the Mach number
$q_c$, the dynamic pressure
$p$, the static pressure
$\gamma$,  the ratio of specific heats (1.4) 


If there is an error in the measurement of either the dynamic or the static pressure both the indicated airspeed and Mach number will be affected. 

Note that the Mach number has no direct dependency on temperature.

The true airspeed is calculated by the ADC from the Mach number and the outside air temperature. 

$$V_{TAS} = M_a \cdot \sqrt{\gamma R T_S}$$

with


$R$, the specific gas constant (287.05 for dry air)
$T_S$, the static air temperature


The indicated true airspeed has a direct dependency on the static air temperature ($T_S$,SAT) also known as the outside air temperature (OAT). The static temperature cannot be measured directly since the air will heat up on impact with the temperature sensor due to adiabatic compression. Instead the total air temperature ($T_\mathrm{total}$, TAT) will be measured.

The relation between the measured total air temperature and the static temperature is given by: 

$$\frac{T_\mathrm{total}}{T_{s}}={1+\frac{\gamma -1}{2}M_a^2}$$

Since the measurement probe may not recover all the energy from the adiabatic heating a correction factor is introduced: 

$$\frac{T_\mathrm{total}}{T_{s}}={1+\frac{\gamma -1}{2}eM_a^2}$$

This 'recovery factor' $e$ is determined empirically for the temperature sensor used. 

The display on the EFIS will show the OAT, which may be slightly off if $e$ is not entirely correct. This will also affect the "true" airspeed indication.

To conclude; anything indicated in the cockpit can contain measurement errors, regardless whether it is called 'indicated' or 'true'. 

With the introduction of digital flight data systems the error correction tables have been moved from the manuals to the computer, which automatically corrects for them.