We know nothing about Joker's parents. For all we know, he frequently communicates with them whenever we are not controlling him.
Not true at all that we know "Nothing" about Joker's Parents. Both Soijiro and Ryuji both talk about how Joker's Parent's "Didn't want any trouble" to not even bother fighting for Joker's sake.
You can make the Argument for Ryuji just being a friend. But Soijiro's words would have more worth as a legal gaurdian. During the course of the story as well as during his own Social link, Soijiro has made mention how Joker's parent's were "Just as bad" as he is when it comes to parenting.
The only thing we don't know is Joker's view on his own parents since as always, Atlus goes with the silent protagonist route. Same goes for P4 Protagonist as well. Any information we get is from 2ndary information like Dojima and the teachers but the Protagonist of any game never reveal information of their Gaurdians themselves cause Mute syndrome.
We also know that Joker's parents are just average people. Nothing to note of them. Granted that could be a set up to plot twist that his parents are scientist or something (like Yukari's dad was in P3) but if they were gonna pull that plotwist, they would have done that already.
