Much like all the Wammy's children, he has a very low percentage when it comes to Social Skills. Matt is not a very social person and prefers to keep to himself. He would much rather be alone.
He is the only Wammy's kid to ever admit that Misa Amane is "cute" Matt is both serious and teasing when he said this. He had only acted this way with Mello. Many times when he seems to be joking, he is dead serious.