Being difficult and unresponsive was probably one of the things Sloth did best. She didn't even have to try, really; it came naturally, like sleeping, or breathing, or annoying her brothers [something that could be accomplished by doing as little as the two aforementioned activities].
"Whatever, man."
Somewhere, on some level, her refusal to cooperate was about more than laziness. Sloth's nefarious intentions were very rarely thought about--she left the evil plotting to the more active of her pantheon--but were more instinct, or some twisted, lazy interpretation of it. Perhaps it wasn't smart to piss Belphegor off [further, though he didn't seem to be acting too irritable right now], but some Sinly thing, buried deep in her subconscious, was purposefully slamming its hand on the apathy button like a trigger-happy child for the sole purpose of seeing if he could be riled up by such petulant, yet harmless behaviour. It would let her gauge the severity of his feelings, for someone who was balancing precariously upon a blade of control was more likely to overreact, while a more relaxed figure would hardly be so easily provoked.
It probably didn't help that he was her demon, and she was known for her ability to instill uncaring in people, or simply make them feel too tired or lazy to respond in a heated manner. Put her and Bel in a room, and you had the laziest room in the whole universe--or, car, in this situation. [She was honestly surprised the driver was still awake.] Despite that rather glaring problem, considering that her very presence was inhibiting the reaction she sought, having a modus operandum was a big enough deal as it was; Sloth didn't really have the energy to second-guess her Plan Of Action, despite the many holes it had.
Her cards were laid out on the table to match Belphegor's own. They were slightly crooked, bent at the edges, and despite it having taken her forever to assemble them, what she was actually getting at remained painfully unclear; still, cards were cards, and a return was a return, even if she had only barely batted the ball back over the net.