If the dipole has its axis vertical therefore
parallel to the pole, it might couple to a metal
pole and distort the pattern and the match, depending
on where it is relative to the pole, and how many
wavelengths it is away from the pole.
If it is
less than one wavelength from the pole, and the
pole passes right past the antenna, then it would
be better to use fibre-glass tube for the pole.
If the axis of the dipole is in line with the
pole, in effect an extension of it, then use fibre-glass
for a mechanical connection to the metal pole.
PVC has an unfortunate tendency to creep out
of shape over time, and degrades under ultra-violet
from the sun, going brittle after a year or so.
So-called u-PVC is better than the ordinary stuff,
but fibre-glass is even better. Wood is OK if you
impregnate it with a polyester wood-preserver.
If the antenna is horizontal and the pole is
vertical, there should be little or no interaction
Lowering the antenna to within
about one wavelength or so above ground has the
effect of coupling it to another "phantom" antenna,
being the electro-magnetic image of it in the ground
mirror. Hence you have 2 antennas in parallel to
put it crudely. It effects not only the impedance
but also the pattern.