I think it was pretty obviously meant as a joke, albeit one in questionable taste, and if the only audience was people who actually knew Sacco, there would have been no problem. The issue is that she tweeted it, complete strangers chose to take offense, and because she was disconnected for 11 hours, things escalated beyond the point of her being able to control the damage. Without realizing it, she threw a match over her shoulder and set her life on fire, and by the time she realized what had happened, it was far too late to salvage anything. You can make off-color or satirical jokes to people that know you, but it's certainly risky to do so in a public forum, where there is no context, and strangers won't bother, or won't be able to determine what you meant, but instead take everything at face value.