One of many instances in the play where the superficial is more valuable than the profound, the expression made by Algernon to his valet, Lane
I do not play accurately- anyone can play accurately- but I play with wonderful expression
is indicative of the shallow nature of the play's comical characters which, in their unique way, mock superficiality in itself as the conduit through which dynamics take place. In not so many words, the characters are meant to mock the seriousness of just about every situation in life and instead focus on the illusory version of it.
In the particular case of Algernon, his piano playing, according to him is worth listening because his intention is for it to be agreeable to the ear, whether it is agreeable or not. He cares very little for the opinion of others regarding his music; he only cares about his own opinion of it, which is likely to be quite high.
Therefore, Algernon's words are the first signs of what the audience is about to expect from him, as a character: he is a conceited upper-class bachelor with a good amount of love for himself and with very little cares in the world, particularly about people or family. His piano playing is only a fragment of how Algernon really feels about those who surround him: as long as it is about Algernon, it is interesting to him.