At risk of Godwin-ing this conversation, i'd have to disagree with you GreatBigTree. No matter how despicable a person was in life, the celebration of their death is still unacceptable. Dancing on a certain German's grave (a little past due, of course) does nothing to change what happened, and serves only to degrade yourself. You don't go digging up bodies to slap them in the face just because you didn't agree with them.
Commuting an unethical act to an unethical person doesn't suddenly make your poor decision better. And this isn't anything so extreme as that. So no, it's not acceptable.
Well, I guess I'd have to ask if you believe that "good and evil" just ideas / perspectives, or if you believe that "Good and Evil" are two forces in the universe that are influenced by the actions of sentient beings?
I happen to believe that there is such a thing as "Good." Most religions seek to increase it, most positive philosophies seek to increase it, most societies seek to increase it, many people seek to increase it. For me, Good is a tangible thing, that can't be measured, but felt. Like the way you can tell a room gets hotter or colder without needing a thermometer, you can just feel it.
If a person believes that another has been increasing Evil, through the suffering they've caused, or other methods of increasing the Evil of the world, then rejoicing in Evil overcome [in this case, through outliving...] is natural, and life affirming. I bear Mrs Thatcher no personal ill will, I've never met her and I'm unaware of any actions she's taken that have harmed me or mine in any way. She seems to have done so to the people in Brittain, though, so to those that outlived her, may you rise above that harm.
To deny or hamper the joy of someone that has endured hardship, and outlasted it's source seems... hmmm... like something that is outside of the denier's scope of influence. I'm not really sure how to express that in a more clear fashion. If someone has wronged you, it is just to seek compensation. In this case, no one harmed by the actions of a powerful public figure will ever see direct compensation for the harm. Taking pleasure in seeing the source of one's pain put underground is a natural extension of the human notion of revenge / punishment, which are [small?] parts of the notion of justice.
There are a few people that I've known, whom cause misery and pain to those they "love". I'm fortunate in that this is not the case for myself, but I have friends and aquiantences for whom I'd celebrate the death of those that harm them. I would not see that as degrading myself. I would see that as celebrating the end of an Evil person, and the potential for the harmed to heal, increasing the Good in the universe.
*shrug* Maybe it is something you need to see from the bottom, to understand the opinion.
PS: Of course Evil is in the eye of the beholder. I draw my line here, Jack draws his line there, and Jill draws her line inbetween. My line is where I feel it should be. I can make judgments on a case by case basis, because I'm an intelligent and mostly rational creature. Everyone has their reasons, and if Mrs Thatcher, or anyone, winds up on the other side of the line, then joy to the survivor. Life is for the living.