Last week I came across two different transformer pads with a considerable amount of mud and mulch piled about 2-3 inches above the pad and around the cover.

I didn't have the cover removed and there were no other readily apparent problems with the installation.

However, typically, I find that outside transformers are fairly well mounted and clear of debris. This was a little unusual (at least for me) and while it didn't look quite right, I can't find anything that prohibits a transformer mounted on a pad from having mulch, dirt, or other material piled up around the cover.

I couldn't find anything in Article 450. Anyone aware of any requirement along this line?

Depends on how much stuff was piled on the sides. Mulch and dirt do not have the same thermal resistivity as air, so the xfmr may lose some of its ability to dissipate heat.