Does the radar altimeter's position affect its accuracy?
In the aircraft I fly, the radar altimeter (aka radio altimeter) is calibrated for the installation, and the calibration compensates for the flight attitude of the aircraft at touchdown.  The radar altimeter is considered radio navigation avionics, and in many cases is required equipment.

So in theory, a flight attitude other than a normal approach attitude may cause some error.  However, since the primary use is during approaches, the consequence of this error is negligible, assuming flaps and gear extended.

In practice the avionics tech verifies that the altimeter reads a certain reading on the ground, and that reading is within spec for the installation. Practically, this means that the radar altimeter does not always read "0" when the aircraft is on the ground in a taxi or parking mode, however it is normally close.

My assumption in answering this is civilian aircraft, using normal low level short range radio altimeters, as opposed to delay Doppler or synthetic aperture radar which are used for other types of navigation applications.