There have been similar ideas as to misinterpretation, however the first one (with taking into account the linguistic aspect) is truly original! It depends on the circumstances, of course, but anything is possible. Nevertheless, I must say that of course it was not only Pippin, Frodo or whoever who would be misinterpretating the Mouth's name here. Given the way in which the Red Book has been compiled, many of the facts have been probably carefully checked by the scholarly minds of Bilbo and Frodo and a mistake like that would have likely been uncovered. If somebody misinterpretated the name of some totally random hill in the wilderness, nobody will mind, but the Mouth of Sauron seems to be a known and feared figure, at least among the "high-ranking" people, all of Sauron's direct enemies probably knew of his existence (probably just as well as they knew about the existence of the Nazgul). In that sense, his name had about as large chance to be misinterpretated as, let's say, the name of Cirith Gorgor or that of Gothmog the lieutenant of Morgul.
If it were to be said that the name "Mouth of Sauron" is a result of misinterpretation, then the variant I could accept - in this paradigm - would be that it was Tolkien who misinterpretated it, resp. as it were, that the Westron got misinterpretated while being translated into English.
