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.