How much of a flop were her later part films?

it's eternally mind bloggling and surprising to me how these films are not even remembered negatively.

That's probably because they went unnoticed and they weren't supposed to be hits. When someone talks about flop they generally refer to movies that are highly publicized with a big budget or expectations which then fail at the box office.
None of her films of that period fit that category.

I think the biggest flop of her later career was The blue bird. It was publicized as this great family-friendly fantasy film, the first picture made in cooperation with the USSR, filmed in Russia, directed by George Cukor. She had four roles in the film, so she was the star.
When the picture was released the reviews were terrible. The quality of the film was mediocre, the film is slow and uninteresting, for either kids and adults. The film went unnoticed and I read somewhere it made less than a million at the box office. I also read it killed Taylor's career even though it had been languishing for years before.
In the previous films at least she still had the beauty and the body. Here despite being only 44 she looks much older and bigger too.
If only Disney had made this film with no Russian input in it.
It is one of her least known pictures. It was made available for purchase only in 2005 almost 30 years after it was released through a Russian retailer. Before that it was very hard to find. Its legacy is that it remains as this unique oddity of that era, not sure where to place it, if in films so bad they are good or just bad films. It's a really bizarre film.

Apart from this one I would probably choose these 'honorable mentions':

Reflections in a Golden Eye (1967)- A thriller a la Tennessee Williams with two big names like Brand and Taylor in their first film together. For some strange reason the film was shot in Rome (!) and using a yellow filter(even though it was released in colour). For two stars of that power, the film failed to attract audiences. It was not an easy film to sell though.

The only game in town (1970)- Her last studio picture and her last Hollywood film. Taylor playing a chorus girl at 38 insisted the film be shot in Paris which blew up the budget. Her leading man was heartthrob Warren Beatty but that did not help the film. It was a box office failure and it pretty much banished Elizabeth to UK/European productions for the rest of her career.

Her film career didn't have one downturn like most great careers, it kind of unceremoniously fizzled.
She didn't have one 'killer' that made a difference, people decided not to care about her movies anymore. She had become too much of a 'celebrity', she wasn't a serious actress anymore, she was the most famous woman in the world.
The years 1967-1968 were brutal for her- She was in 6 films in that period and only one of them made money (Taming of the Shrew). There's only so many failures an actor can take, even Elizabeth Taylor. Hey at least she had the fame, another actress in that circumstance would have been stuck on TV from then on.

I guess the only similar case to that would be Brando.
I wouldn't be so sure, Brando's career had its ups and downs. When everyone thought he was done he came back with Godfather and still made some interesting appearances in big films of that decade like Superman, Apocalypse Now and at least his experimental or avant garde films were talked about like Last Tango in Paris. Taylor's career in the 70s has nothing of that. Even Brando's last roles were in A films co-starring big names like Johnny Depp or Robert de Niro At this stage he was feeding off more from his legend than his acting but they were still wide releases. I wish Taylor had had this, but it wasn't to be.

I wish she had had that too, but that's typical sexism, film is male dominated and no one was dying to cast her, almost all films produced are male stories. She was thin in The Blue Bird!

Agree, she was slender in the Blue Bird

What happened was TOO MANY films, too many with Burton, and too much off-screen publicity.

Her public--which was aging, along with her--had to make a choice between spending money and time seeing her movies, or just reading about her in fan magazines. At that point, there didn't seem to be much difference between what she did on screen and what she was supposed to be doing off-screen. (And her attempt to be a normal "girl" in "The Only Game in Town" made her ridiculous. At least her other latter-day films presented a recognizable Liz Taylor, enjoyably over-the-top.)

Interest in her films just faded away, but--incredibly--interest in her didn't. That is an astonishing aspect of her life as a star. Usually, once the box-office goes, so does media/public interest. But in a weird way, she became more famous, not less, as her career meandered off. That's the reason why studios were still willing to spend a lot of money to get her ("The Bluebird") and why she was still in the running for quality projects like "A Little Night Music."

There was talk of her for the Anne Bancroft role in "The Turning Point" and several other things before she went down to the farm with John Warner.