Western society does not have explicit castes, and even notional/nominal class is tied now more to education and profession than to birth. In fact, it is more than a little bit politically incorrect to talk about these things, because it condemns people for things beyond their own control.
If you are talking about somebody who’s nobody special, you might try for an average Joe or Joe Everyman. This is a bit less unkind than calling a member of the masses a “nobody”.
Moving into strictly pejorative territory now,
per the OED, a lowlife (irregular plural lowlifes instead of *lowlives) is “a coarse, vulgar, or no-good person.” Inspecting a thesaurus will lead to countless synonyms of such good-for-nothings and scoundrels.
Similar queries can be constructed if you are more interested in focusing on them as uneducated and uncultured know-nothings on the one hand, or on the other, as the poor unfortunates and related do-nothings who make up the unproductive non-working class, sometimes called the permanent underclass. In earlier times, and perhaps even now, these unscrupulous cads were also called stick-at-nothing, for which the OED gives the example of “a false, lying, swindling, underhand, stick-at-nothing brute.”
If there is any connection to be found here amongst these many pejoratives, it is the “nothingness” link connecting good-for-nothing, do-nothing, know-nothing, and stick-at-nothing.