I don't know what to say about this.
He looked pretty serious to me in the last fight with his brother.
Even though he might have that funny and sarcastic façade,he knows when to be serious and when not to.
I think he puts that behaviour in order to preoccupy himself or to forget about the feeling of loneliness he has,because Dante is a lonely guy after all. (How do I know? Because I'm lonely too and I try to put on a smile to forget about it too)
But when it comes to family matters,he acts serious,especially when it comes to his brother wanting to stay in the demon world.I didn't see him smile or joke in the last fight,I think that was when he changed into a more serious person.
He is seen saying 'soul' many times in the games,that's why I think he puts his emotional and rational side in the first place compared to his brother.Again with the theory Dante represents Sparda's soul and Vergil,Sparda's power.
The fact that he is joking around powerful demons is because he is confident and they don't have anything to do with the persons he cares for,like his family or friends.
DMC 4 Dante isn't exactly my favourite,I mean,yes,he had some good jokes and Shakesperean acting,but he felt quite an...empty character.