Speaking for the Buro fur Grammatik-Schultz und Durchfuhrung, this is not exactly wrong - it’s a meme, in which the vital comment is grammatically incorrect. Indeed, one could argue that the ungrammaticality of the “SON I AM DISAPPOINT” is what makes it stand out and become memorable (and, thus, a meme).
Further, to propagate the meme, one MUST maintain the original format of the statement/caption/dialogue, thus any errors must remain. It’s akin to a newspaper reporting what someone said ‘on the record’ - if someone uses an adjective instead of an adverb, or uses the object ‘me’ instead of the subject ‘I’, the newspaper is still required to print the quote uncorrected…and they’ll tag a “[sic]” after it to indicate the error(s) is(are) of the original speaker, not the newspaper.
So, despite its essential wrongness, “DISAPPOINT” has transcended the standard rules and become something of an new format of interjection. And one must remember that a language must innovate and evolve and adapt over time, else it withers and dies.
…oh, and you forgot the quotation marks that should be around ‘Disappointed’. :P