As TV Tropes explains:
"Of course, in addition to Swiss and Limburger just being funny, there's a practical reason behind this trope as well, which is based on the Rule of Perception. In a cartoon, a large, nondescript block of something yellow or white could be just about anything. A bar of gold? Butter? Lard? A white brick? Giving cheese these distinct traits helps identify to the audience that "Yes! This is cheese you're looking at." Of course, this works best if you don't name the cheese in question.
Commonly seen as part of a Level Ate."

The cheese is that good, that the Pikachu wants be as close to a cheese shape possible, so he ate it WHOLE with a help of an assistant. He starts waddling away turning from satisfied to a shyness looking to see the shape he is now. ;P