It seems popular to discuss how Beethoven found Diabelli's theme, from which he composed his thirty-three variations, a botched piece of cobbling. The theme itself is actually quite catchy and Beethoven was nothing if not irascible and prone to resentment. While I suppose those who insist on the more romantic idea of Beethoven writing furious rejoinders to Diabelli's theme to the tune of 33 rejoinders is the best way to understand it, as a composer, I feel it is necessary to point out that you don't write THIRTY-THREE VARIATIONS on a theme that you really hate. Bach certainly wasn't against writing so many variations in his Goldberg set. It may be useful to puncture the balloon of Beethoven composing a set of variations in an angry response to the work of some lesser artist. The story of Diabelli's theme earning Beethoven's ire can only be seen as some romantic fantasy on the level of Mozart as depicted in the story of Amadeus, transforming a historical figure into a cipher through which to complain or denigrate perspectives in the arts you don't like or as an allegorical figure depicting a point you wish to make.
Now, to be fair to this concept, I suppose I should mention that I have listened to Diabelli's sonatas for solo guitar, F major, A major, and C major respectively (not necessarily numbered in that order). Certainly compared to Beethoven's piano works the guitar sonatas will seem slight and obnoxious to many. Even compared to Haydn, who is often considered a polite courtly composer without a lot of profound things to say (a pretty grisly misrepresentation of the master whose work and life inspired both Mozart and Beethoven), Diabelli does seem to lack a bit. The reason I can grant this dismissal is because the big three Classic era composers were able to accomplish something Diabelli is distinctly not able to do, to compose expositions that bear repetition before proceeding to the development. With all due respect to the formal conventions of the time Diabelli's expositions are okay but they don't warrant the repetitions that historically informed performances would go with. Sad to say that but Diabelli's expositions are not at the level of the Fifths quartet or Op. 111 or the Jupiter Symphony.
Of course ... the whole other point is that it's grossly unfair to find a guitarist-composer's work wanting in comparison to such titanic works in the Western musical canon. Even so ... Diabelli's expositions are not as compelling as Sor's expositions within the same period (they were born and died within the same period).
Beethoven's music is full of pretty jovial material if you've heard it, which is another reason I dismiss the notion that Schindler's tale has any weight to it, the whole Beethoven writing in furious parody. Parody is certainly present but here we should make a point of discussing how parody is a compositional technique (i.e. in the "parody" mass in which a secular tune is used for a sacred text-setting) rather than the more literary meaning many would assume. Parody can be both jocular and completely serious and perhaps the best example of this, aside from Beethoven's own set of variations on Diabelli's theme, would be Charles Ives' use of parody and quotation in his violin sonatas. But to get into that is more than I feel like doing at 2 in the morning in the middle of summer!