My own strong suspicion is that this is all about Maresca "explaining" how he had a brilliant legal stratagem underpinned by his brilliant legal mind, which would have resulted in a successful outcome had it not been for the malign and disruptive influence of his clients, the Kercher family.
I think that Maresca's reference to the Kerchers as "lower-middle-class" (and I think that's his meaning) is simply a proxy for what he'd like to say - which is that (in his opinion) the Kerchers lacked sufficient intellect and education to enable him to do his job properly in this case. The same rationale goes, IMO, for his coded reference to JK's alleged alcohol dependence problem, and for the internecine and intra-familial disputes among the Kerchers (which do have supporting evidence - for example, I think it's true that for some time now, Stephanie Kercher has been operating more-or-less separately from many other members of the family wrt this case).
In other words, IMO Maresca is effectively accusing the Kerchers of having got in his way - through their uninformed, ignorant and ill-educated meddling, their dysfunctional family structure, their character defects and their in-fighting - and thus hampered his ability to act properly on their behalf and on behalf of Meredith Kercher. The flip-side inference (of Maresca's) would be that if the Kerchers had been decent, intelligent, well-informed, well-educated, united, thoughtful, deferential (to him) and unafflicted by substance abuse issues, he (Maresca) might/would have been able to deliver "justice for Meredith"..........
Aside from the fact that some of what Maresca has to say about the Kerchers rings true, the fact that he stoops to say it unveils him as exactly the kind of classless, blame-shifting weasel one had the intuition he was, all along.
All of this just confirms what a disgusting circle of ghouls this case elicited. Perfect revelation for our current season, in fact.