I completely agree with your assessment of the Kerchers' speech and accent characteristics. But I actually think a native Italian with little immersion in the finest nuances of British accents (e.g. Maresca) would almost certainly not be able to detect those nuances in the Kerchers. Rather, I very much suspect that Maresca's (pejorative and snobby) dismissal of the Kerchers as "lower-middle-class" was based on the way they acted in front of him, and the way in which (perhaps) they were unable to grasp some of the more intellectually-challenging aspects of the case and of Maresca's trial strategy.
I strongly suspect Maresca also knew things such as what kind of houses/neighbourhoods the Kerchers lived in, and what levels they'd attained in education and employment. Obviously he would also almost certainly have known about how much disposable income they did/didn't have, and what levels of capital assets they had. All of those indicators would have informed his view of the Kerchers' demographic classification. As it happens, he's probably pretty accurate. As you say, their accents are a distinct pointer to anyone who is sufficiently attuned to British accents and vocal cadences. And the (for want of a better term) patriarch, John Kercher Sr, was a freelance jobbing hack (certainly not editorial-level journalism, nor anywhere near).
And, as I said, my view is that Maresca was frustrated by the Kerchers to some extent, and he chose to take out his frustration on snobbish character assassinations based on his assessment of the Kerchers' social class. In addition, I think it's entirely possible that any genuine frustration/obstruction Maresca might have felt in reality has been significantly magnified for the purposes of this book. After all, Maresca has a reputation to protect, and a fee-paying existence to preserve. And therefore it's easy to see how a narrative along the lines of "I'd have won this case if it hadn't been for the pesky, bickering, class-deficient clients I was lumbered with" is meant to convey the following implied message: "I, Maresca, am a brilliant lawyer. You cannot pin the blame for my failure in the Kercher case on my lack of skill and talent - the blame in this instance lies with the client. So you, my well-educated and erudite legal friends and potential future clients, should continue to have total faith in my abilities and chances of success."
In other words, the Kerchers might not have been ideal clients for someone such as Maresca - but there's a logical reason why a rat such as Maresca might choose to magnify the Kerchers' shortcomings and adverse influence, in order to pin all the blame for his failure upon them, in order in turn to try to protect his personal reputation and future earnings.
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