Well, throughout 2012 she was equal third best on tour in terms of the percentage of points won on second serve, so I don't really think there has been a massive drop off in terms of her physical ability to hit the shot.
She's lost some confidence - might take a while to rebuild her feel for it.
I think it's probably mainly a confidence thing, and tbh since her first set against Safarova in the FC she has only hit about 5 or six DFs in six sets (including her doubles yesterday), so I don't think it's anything dramatic.
In general I think she's going for variety on serve these days anyway, rather than throwing in a huge kicker every time. It stands to reason when you're varying your line, pace, spin, etc. a lot you are more susceptible to hitting DFs
I love how convenient it is for Serena fans to mock any player of their choosing simply because they're not 'The Lord!11!1!!!'. Fact is, Sam's second serve still is one of the best in the game, and while she's lacking confidence right now, it's ridiculous to say that it's 'overrated' when it was a large contributing factor in her successful slam runs. And as stromatolite said, it's obviously mainly because she's trying to mix it up more, and not because she's trying to 'over-compensate.'