I think the way she went about it was rude. I think she should have asked permission to plug her phone in, said something like "I am so sorry but I really need a charge in my cell phone because [insert reason here], and that is the only electrical outlet I see, would you mind if I used it please?". And then given the OP the chance to get up and move out of the way, or plug it in herself or something.
That said, it probably would have been best to tell the restuarant staff of her plight and ask where would be a good place to plug the phone in. They might have had a better solution, under their counter or something. Not that I think they would be obligated to provide this service but if it was an important issue for cell phone lady, they might have been able to accomodate her.
And who knows what her reason was? It might really have been something important.
So yes, I think she approached it in a rude way but I think I personally would just easily let it slide, because who knows what was going through her mind. She might have been in a really important need to charge her phone, maybe so important that her politeness meter was a bit scrambled.