It is false that for visualizing the relationship between a numerical and a categorical variable, a mosaic plot is useful. This is because the best plot to use for this kind of relationship is the side by side box plot.
Comparing the distribution of a numerical variable across the levels of a categorical variable is something we usually consider in this type of relationship.
The levels of one categorical variable by means of a quantitative variable is best compared by using the visual display called the side by side boxplot. By placing single boxplots adjacent to one another on a single scale, the side by side box plot is constructed.