A common trend that I noticed is that the tower can only be brought down with physical damage. Towers appear immune to any other types of damage, namely magic and true damage. Of course as we all know, we can't cast spells that deal magic damage on towers. And any on-hit procs that deal magic/true dmg is also negated when using against tower like teemo poison(magic damage), twitch poison(true damage), etc.
The ONLY two exception to this is Rammus's ult which deals magic damage and Alistar passive which also deal magic damage. However in both cases they explicitly explain in the skill description that it will affect buildings.
However, on-hit procs that deal additional physical dmg such as wukong's q does affect tower. Also note that Lichbane also affect tower because it is bonus physical damage.
So since Pix's damage is magic damage, it will not affect the tower in any form regardless who Pix is currently following.