What is meant by "mushroom grandeur" in this context?

What is meant by "mushroom grandeur" in this context? Is it metaphorically means instant, like how mushroom blooms?

"What do you think of that?'' asked his expressive little eyes, when something cruel in "mushroom" grandeur or crude style passed by.

-Ceylon, the Paradise of Adam By Caroline Corner, pg 45-

A little more context from your source:

    “What do you think of that?” asked his expressive little eyes, when something in “mushroom” grandeur or crude style passed by. Bad form he could not endure, nor vulgar taste.

Compare:

6.b. attrib. quasi-adj. with the sense: Resembling a mushroom in rapidity of development or growth or in brief duration of existence ; upstart ; ephemeral.

[among citations reaching back to 1599:] 1887 Westm. Rev. June 269 Radical millionaires, and mushroom aristocrats.

—OED 1, s.v. Mushroom

Punch despises the tasteless grandeur affected by nouveaux riches.