Roald Dahl conveys the importance of the pumpkin in Lord Emsworth's life by doing two kinds of juxtapositions.
First, he introduces life at Blandings; the reader learns about the history of the castle, the importance of Lord Emsworth's social position, the amount of money that the family name represents, and about the overall charms and grandiosity of being an upper class family with royal issue.
Dahl then juxtaposes this to the shallow, even hollow, intellectual nature of a man who has so much money to do so much, and yet naturally limits himself to accomplish so comparatively little.
The importance of the pumpkin the Earl of Emsworth's life requires, perhaps a word of explanation....and meanwhile he would go up to London and engage a real head-gardener, the finest head-gardener that money could buy.
Dahl even adds a true human dimension to Lord Emsworth daily existence: the fact that the life of Lord Emsworth is not devoid of real problems. His son, Freddie, is idle and clueless. He is problematic in terms of how he interrupts the boring dynamics of Emsworth's life, causing Emsworth to potentially find the need to worry about something important- which is unthinkable.
Another interesting and realistic fact in Lord Emsworth's character is that he is a misfit among his peers; his lack of intellect and sophistication render him an oddity in the upper class circle as well as a true waste of time and money: even with his riches, and with the opportunities that his rank offers, he could not care less for London, the West End, his aristocratic peers, and maybe not even too much for his son, who is merely a nuisance in terms of how he interrupts Lord Emsworth naturally bland life.
The fact that Lord Emsworth is a careless, clueless, and oblivious aristocrat whose myopic world allows him to worry about truly unimportant things, makes him a satirical characterization of people of his class: a small fragment of society that is well-known for their lack of intellectual attainments, and for a penchant for wasting the vast amounts of time and money that they have in their hands.