The Yams seller is a minor character who appeared in several episodes of the series (especially at the beginning, although he has sporadic appearances later).
He walks through the streets of Kasukabe with his truck full of freshly made yams. However, in several episodes he sells yams to other people (like the Saitama Crimson Scorpions), but instead of in a truck he sells it in a typical wooden cart which is about the size of a bicycle.
Since the sweet potato is a starchy tuber and produces gases, the ladies who like it are ashamed to be seen buying them. So Misae sends Shin chan to buy them and Ume also hides to avoid being seen.