Joan loves seeing him content and happy, especially with everything that's been going on. Joan herself has never really gotten into cooking. While she certainly enjoys food (unlike Sherlock, she never sees it as merely fuel) she's never really dedicated the time to cooking. She usually has other things she'd rather spend her time doing, so the takeout restaurants nearby are very familiar with Joan and Sherlock, as are the takeout restaurants near the Brownstone back in New York.