Stephen's questions shows that we cannot apply any concept to any object. For example, a chair in itself is neither tragic nor comic. It could become so only if the artist changed it, in order to stand for a tragic or comic representation.
The second of Stephen's questions relates to the juxtaposition of the terms beauty and good. Good could imply here "finely made", therefore beautiful. But he adds the term "desire", which could mean "esthetic desire", an oxymoron in his view: esthetic being a static emotion, desire being a kinetic one.
The third of Stephen's questions regards the way in which a work of art is produced. Can a work of art be produced by accident? I think yes, as long as the "author" realizes that wanting to do something else, he came across something that can be regarded as a work of art.
The image of the cow is not randomly chosen: for Joyce, the cow is the symbol of Ireland.