The overriding theme of the poem is appearance v reality. On the surface, the bourgeoisie, especially the men, appear solid, respectable, decent, and "eminently presentable." But if you look beneath the surface it's a different story entirely: the bourgeoisie is rotten to the core. The bourgeois male is so stuck in his ways that, presented with any new life situation, any moral quandary or challenge to his limited understanding, he simply cannot cope. And so he goes all weak and soggy like a big wet meringue.

The bourgeoisie are supposed to be Britain's backbone, the very foundation of national strength. But Lawrence despises them for what he sees as their inherent weakness and decadence; far from being the nation's backbone, they have no backbone at all. And so they compensate for their lack of vim and vigor by feeding parasitically off the "dead leaves of greater life than [their] own." This is possibly a reference to the unthinking jingoism of the British bourgeoisie, hankering after an idealized history that never was. The bourgeoisie have no real present and no future, so these "mushrooms" as Lawrence caustically describes them, must look to the past for moral sustenance.

The central theme in "How Beastly the Bourgeois Is" addresses hypocrisy and corruption.  D. H. Lawrence criticizes the "male of the species" for having the appearance of goodness and the substance of a "toadstool."  According to Lawrence, the bourgeois male flourishes in simple times when life is successful and pleasant, when he can "go tramping thirty miles a day after partridges, or a little rubber ball"  (7-8).  Yet according to Lawrence, the bourgeois male cannot stand up to difficulty, and Lawrence's poem adopts an extremely critical tone to criticize these shortcomings, comparing the bourgeois to a floppy "wet meringue" with no substance. 

"How Beastly the Bourgeois Is" attacks the lack of substance found within this particular group.  Lawrence's poem criticizes their appearance of strength and goodness which so easily falls away to being a  "mess, either a fool or a bully" (15).