That contrast is definitely jarring! It feels like a huge romantic gesture is being treated like a grocery list.
The way Kay (Mrs. Miniver) "forgets" and Clem (Mr. Miniver) brushes it off is often interpreted by film critics as a tool to show their humility and the breaking down of class barriers. To them, they are just ordinary people; they don't see themselves as "icons" worthy of having flowers named after them.
Here are a few ways to look at that specific scene:
The "Stiff Upper Lip" Tropes:
In 1940s British cinema, being overly emotional about one’s own importance was seen as "un-British." By having Clem worry about a bathroom light instead of the rose, the movie emphasizes that they are grounded, everyday people—the very people the audience was supposed to identify with during the war.
Transition of Stakes
Some viewers see this "forgetfulness" as a pivot point in the film. It marks the transition from the "lightness" of pre-war concerns (like flower shows and expensive hats) to the literal life-and-death struggle of the Blitz.
Symbolic Humility
Kay’s casual attitude makes her later bravery with the German pilot feel more authentic. If she were the type to brag about a rose, her quiet strength in the bomb shelter might not have landed as effectively.
The Downton Abbey "homage" to this plotline treats the competition with much more gravity, which might be why the movie's more casual take feels so "buggy" by comparison.