I’m actually 100 percent positive that were Oprah on the Supreme Court she would do a good job. In a lot of ways, it’s just not that difficult a job. You need a reasonably intelligent, public-spirited individual who’s aware of their own limits and does a good job of hiring clerks. To be a truly great justice requires more than that, but it’s not as if putting a TV personality on the court would lead to her making “wacky judicial bloopers” or something. The difficult, controversial cases that come before the Supreme Court are precisely the cases where the answer isn’t in your bar exam study book.
A few thoughts. One, take a look at the Supreme Court's docket this year. Most of the issues it confronts are, in fact, highly technical questions of law (see here, here, here, here, and here, to choose a few examples) that do actually require judicial expertise. Though, truth be told, I kind of would like to hear Oprah's thoughts on whether the expert witness attendance fee provision of 28 U.S.C. 1821(b) applies to cases within the Supreme Court's original jurisdiction in addition to the cases it hears on appeal.
Second, it's true that a handful of the cases the Court hears do concern controversial political issues on which the relevant legal test is an ad hoc balancing inquiry in which Oprah's views would carry as much weight as anyone else's. But, even assuming these are the only cases one cares about, I would be hesitant to conclude that therefore a non-lawyer could serve just as effectively. Under what theory of judicial legitimacy would the public be expected to accept Oprah's pronouncements on these questions as binding the hands of its elected representatives? The advantage of having boring old judges with fancy legal degrees on the Court is that it helps take these essentially political questions and makes them seem like legal questions requiring expertise--hence the elaborate edifice the Court has built of tiers of scrutiny, fundamental rights, and all the rest, disguising the very un-technical value-weighing going on beneath.
One can argue, of course, that it would be best to strip away that edifice and reveal the Court's jurisprudence for what it is, and I can think of few better ways of achieving that than appointing Oprah to the Supreme Court. But that would be an odd position to take for those who want the Court to actively police legislative intrusions on individual rights. If it became clear that judicial expertise is largely irrelevant in this realm, the public probably wouldn't respond by suddenly developing an appreciation for the theory of partisan entrenchment and recognizing the legitimacy, as a check on the whims of temporary democratic majorities, of judicial appointees voting in accordance with the views of the president who appointed them--which Oprah could do just as well as anyone else. More likely, people would start asking why a Supreme Court with Oprah on it should have any say on matters of public policy at all.