I see it as part of a general decline in Joan's fortunes. At the start of Mad Men she was the all-powerful office manager, able to boss/bully the other women around, mistress to the boss, Roger, and able to give Peggy a run for her money.
However, over time her influence in secretarial politics has waned, with even the upstart Jane being a potential threat.
She showed that she has brains as well as beauty when she was given the chance to work on the TV script/advertising slot material, which was then unthinkingly denied to her.
Her partner was clearly unsupportive of her work when they were shown eating together, and he thought nothing of ordering her to get him a drink of water.
And in the latest episode there is that dreadful sex scene in the office.
At the end of the episode Peggy is shown moving into her new office and musing how to have the name on the door changed, and Joan just says she will attend to it, without any fight or cant or spirit. It seemed to me that Joan had transited from all-powerful woman in the office to someone who'se hopes and dreams had been cruelly crushed on several fronts, and who had had all the fight knocked out of her.
Which is incredibly clever scripting and acting. At the start I rather hated Joan for being such a hard bitch, whereas now I'm really sorry for her. Nicely done.