If the white paste is 'firm' and semi solid, it looks as it could be some sort of potting compound.
Potting compound is often used to fix and hold components in place, also to reduce the electro-mechanical 'buzz' from transformers.
Note: heat sink compound can also be white [ mine is ], it remains sticky for many years but can eventually dry out.
If it's rubbery and doesn't make a mess, then it's definitely some sort of potting, meant to reduce vibration or increase sock resistance (like the blob in the bottom left of the photo).
If it's greasy and makes a mess, then it's thermal compound.
The stuff on the transformer could be either or both. Many potting compounds are made to have good thermal conductivity.
thermal compound with metal is silver, thermal compound with ceramics is white.
If the electrical conductivity of metal may cause problem, then use ceramic compound. Usually the metal compound has better heat transfer performance..
From the impression in the top surface, and the amount of the stuff, I would say that is certainly potting compound and not for heat sinking. It does carry some heat of course but not particularly well. I would say it is more for mechanical stability, to cushion the parts if the PSU is dropped.