Not understanding something has no direct bearing on it's station as nonsense.
A lunatic could ramble on for hours about things that made perfect sense to him but was gibberish to everyone else. Most people would call his ramblings nonsense, but if we're going to argue semantics, there is no such thing as nonsense, just as there is no such thing as pure, human-made randomness. Everything has a method to its madness. We as a society have to draw the line as to where sense ends and nonsense begins.
Do you understand every last word of the Love letter? Parts of it were made to be understood, and other parts were made to be completely alien, nonsense if you will. Certainly it's not just mismatched letters that don't make words, but a lot of those words, in classic MK fashion, are new and have not, prior to his writing the LL, not been seen. I could scour the LL for every last example of them, including examples of new terminology, such as "Digitals" that hadn't yet been a part of TES literature, but I won't do that. You know what I mean, and until corroborating sources were written with some of those same terms, the LL was an island of sorts.
So no, it's not nonsense. Nothing is nonsense. The LL though, with its high degree of foreignness must seem like nonsense either in its entirety or specific parts to all but its creator. And there are a great many people, I would think, who say they understand it but really don't.