I think this poem is meant to address the materialism, pomp, ritualize "convenience", and impersonal feel of a traditional funeral. Williams himself takes a light, facetious tone, but the underlying themes are critical.
Instead of iring a polished hearse with glass windows and flowers, Williams posits a different approach. He suggests that one should, rather than riding above a coffin in a fancy hearse, drag the hearse. No, in fact, remove the hearse altogether. "let it be weathered/like a farm wagon", he says, showing that he prefers genuine simplicity to frippery.
He playfully mocks the use of glass "My God-- Glass, my townspeople!/For what purpose? Is it for the dead/ to look out or for us to see/how well he is housed...". In this stanza he is trying to enlighten his townspeople to the ludicrous nature of having glass windows for a dead man.
He asks for no hothouse flowers, because he sees the flowers as impersonal. One would be better set to display "some common memento" of the deceased. This would be more personal; more fitting.
Toward the last stanza, the tone becomes more serious. the meaning of this passage is that one should not hide grief or suppress it. He acknowledges that grief must be dealt with,a major theme of this poem.