At the club, he was telling Lilith to 'get the f.uck down here' but that was out of anger. The after he defeats her, he's not as cocky as when he killed succubus or Bob, but I guess that's because that part of the plot was about a hostage situation, so he needed to be more serious.
I don't think he loses it, more like he just tones it back, does a little less swearing. He still comes out with things that are snarky, but he chooses the right time to do it. It seems like the bit where he's taunting Mundus, it's like he had to force himself to do it just to get Mundus angry. So, maybe it is a sign of him maturing...
To me he doesn't lose anything is just that his character constantly flip flops between sardonic, cocky, and crude to serious and sympathetic sometimes even caring and nice when the plot demands it.
He walks into a Night Club to get Lilith so he can rescue Kat but first uppercuts a bouncer, writes f*ck you on his board, and then drops it on him while saying a joke all with a
stupid confident grin/smile on his face then the moment he walks into the club he is all serious and angry especially when he meets Lilith and then proceeds back to the jokes and one liners (Barbie, only a face a mother can love). Lets not forget during his fight with Bob when Bob shows that the Order where being attacked (Kat and Vergil is in danger) he is very concerned and worried but after beating Bob this aspect of worriness seems absent as he was able to shoot one more corny one liner and even walk out very slowly after beating Bob almost as if his worries of Kat and Vergil being in danger alleviated from him.
It seems Dante's character and character progression throughout the game has been rather inconsistent or poorly managed. Certain scenes thrown in there to just make Dante look cool when he needed to be serious and scenes were thrown into there to make Dante sympathetic and nice despite him being a prick not too long ago.
Lets not forget certain aspects of the story that is heavily stressed upon on Dante such as Phineas warning Dante that Mundus's wrath could lead to a lot of human casualties but after Dante defeated Mundus as a result of Dante angering Mundus the city was destroyed and there was many human casualties but nor Dante or the plot even reflected upon the damage and human deaths that Dante and Vergil was technically responsible for (or better yet doesn't recall Phineas's advice about Mundus's wrath and the casualties that could arise) but want to know what he and Vergil did (well Vergi don't really count in this)? Dante bragged about who did the most work and laughed it off and for some odd reason Dante wants to be mankind's guardian despite Mundus being right about him causing nothing but destruction and death and being very nonchalant/unsympathetic about the human casualties that he caused.
Nor did the story made any attempt to even make the human casualties a factor to the aftermath ignoring Phineas's prophecy literally making that one aspect of the story meaningless. People still died (there were corpses in the office/buildings when you walked through them in Mission 19 and lets not forget the amount of people being dragged into Mundus along with the debris) for its hard to believe there would be zero casualties in all the damage that was done.