Again, Cassandra Clare is so stupid that she wrote this in and doesn’t seem to care or realize that this is making Clary look like a horrible person. I don’t think that Clary and Jace were necessarily outright flirting with each other, but there was there underline subtext that they were still arguing over their forbidden love and the fact that they are siblings.
And they are doing that right in front of Simon. It’s just so gauche and disrespectful as fuck to Simon. Especially, after Clary cheated on him with Jace. This shit really further chips away at the believability of Simon and Clary’s relationship.