The use of an award-winning vegetable (although really a fruit), as the sole source of interest in the life of Lord Emworth is Roald Dahl's way to use sarcasm when exposing the worthless waste of time, money, and intelligence that the Earl engages in day after day. The pumpkin comes...
The use of an award-winning vegetable (although really a fruit), as the sole source of interest in the life of Lord Emworth is Roald Dahl's way to use sarcasm when exposing the worthless waste of time, money, and intelligence that the Earl engages in day after day. The pumpkin comes later in the story because Dahl has to juxtapose its worth to the net worth of the Blandings, the rank of the Earl, his position in society, and his intellectual acumen. Therefore, Dahl first presents the grandiosity of aristocratic life only to deflate it by making his main character, a patriarch by right, into a dull, myopic man who can only care about worthless things.
Although Freddie, Lord Emsworth's worthless son, causes him a lot of grief, by being as equally asinine as his father, the actual emotion in the Lord's life comes directly from the pumpkin. The devotion that he gives to the pumpkin, as well as to the pig that he also takes to competitions, gives a very good indirect characterization of the Earl of Blandings.
First, we realize that he is, overall, very shallow. He is terrible at prioritizing, and places importance on trivial things: his vegetables, the pig, his flowers, and his naps. As a social commentary, this is a direct satire of the upper, aristocratic classes in England who lived too oblivious of the world outside of their own. Literature of the Victorian/Edwardian periods are rife with characterizations quite similar to that of Lord Emsworth's; it is clear that the writers of the generation do want to send out a general message of how different social class makes its members.
As of whether the pumpkin retains its importance, the answer is that it does. In fact, it is not hard to correlate the pumpkin to the ultimate good news that come to Lord Emsworth at the end of the story: there may be a chance for his son end his idle life thanks to the intervention of powerful acquaintances that became involved in the story when Lord Emsworth lost his common sense and wandered the streets of London in search his gardener. Hence, the pumpkin is ultimately the agent of change in the story: it is the need to care for it that caused the crisis in the life of the Lord; the fact that such care was safeguarded brought with it good changes that may mean a lot to someone as limited in focus and ambition as Lord Emsworth.