Short answer, no.
There are a few rumours on Twitter at the moment that Dolly mimed during her Glastonbury performance. There are a few things I'd like to point out, because rumours like this are only slightly less annoying than people being all 'OMG, does Gamu really sound like that?'.
1. OMG, but it totes looks like she's miming.
And you're watching on the BBC Coverage? Yeah, it does look like she's miming, because there's a glitch in the sight and sound syncing. When I watched it on the live stream, no such glitch appeared, in the edit, they've bungled it and she's out of time. It's actually really annoying if you're watching it, and I'd argue the BBC should have better production value considering how much money they spend on Glasto. The tell-tale sign of this, is the fact that Dolly's also out of time whilst speaking, and her backing singers are out of time.
2. OMG, but it really looks like she's miming!
Notice that she very often uses two microphones at the same time. During this performance, Dolly had a gold handheld mic, and a flesh coloured mic attached to her earpiece. My guess would be that the flesh-coloured one was the one she was using the whole time and the gold one was a prop.
3. OMG, but it actually looks like she's miming.
During the two parts where Dolly used instruments, I believe the instrumentation *was* mimed. The Benny Hill sax skit, and the fiddle piece probably were mimed. I do think she can play those instruments, but I also think she's a professional who wants a tight show.
4. OMG, was she miming though?
You'll also notice that if she *was* miming her vocal, the blends between her speaking and singing were incredibly well done. On the balance of probability, I don't think that her Glasto set was so well planned and rehearsed on the main stage that they were able to blend in and out between spoken and sung word that quickly.
Okay, so I'm being extreme in responding like this, but accusations like this really annoy me, because people so often know so little about lip-sync. It annoys me that a pro like Dolly would be accused of these things, when people like Katy Perry (who I'm also a massive fan of) uses forms of mime ALL THE TIME. So, there are a few versions.
1. Full mime. This would effectively be a singer not singing at all and 'miming' to a backing track - either the original track, or a *live* version recorded in a studio. A good example of this would be this performance by Delta Goodrem when she opened the Commonwealth Games. It's a *live* version recorded beforehand and slotted in with plenty of reverb to give it an arena feel. I imagine they did it because it'd be pretty mental for anybody to sing in the middle of those firework dancers.*
2. Part-mime. This is something really often used for big performances used on TV shows or awards ceremonies. Often, artists will perform along to a backing track where their vocals are included in some parts or all the way through. So, if you're Madonna and you're going to be doing a crazy routine, you might have your recorded vocal overlaying your live one, so that you have some breathing space if you're missing words out. An example, I'd say is this performance of Roar by Katy Perry on the X Factor. I'd argue that she's singing live and unassisted on the verses (note at the opening) but that her recorded vocal comes in to overlay her on the chorus. I don't blame anybody who does this really, especially with big production numbers.
3. Totally live. Like this performance of Blue Smoke by Dolly on American TV. I'm pretty sure she's totally live, you can see how she drops a few notes here and there, you can hear her breathing - things you could hear during the Glastonbury set too.
So, basically - don't diss Miss Dolly. There, I said it.