He probably pointed out that they looked similar because Stryver asked one of the two accusers after being given the note, if he had ever seen someone that looked like Darnay before.
We can infer that it had something to do with their appearance.
And you could have mentioned that this is from "A Tale Of Two Cities"
If the main point is about something that is not funny then it will distract the reader but its about all the same topic it will make it interesting to the reader
