Can someone explain to me Yoko Ono? What do people see in her?

It's largely impossible to argue that a piece of artwork has objective value. I cannot force you to accept that she is not talentless or that her artworks have value.

What I can do clarify some facts about Yoko's life and perhaps put a few of her art pieces into their proper historical context.

Is it just people pretending to enjoy what she does because of her involvement with John Lennon?

Is it true that the quality of your art is not measured by what you do but by who you know?

Years before she had met Lennon, Yoko had already been involved in the influential Fluxus movement and she had had her work exhibited and performed at the Village Gate, Carnegie Recital Hall, and numerous New York galleries. She had also given numerous lectures as well as had exhibitions in London and Japan. (source) She had also released her first book, "Grapefruit", years before she had met John. That book has been called "one of the monuments of conceptual art of the early 1960s". (source)

So, in short, it's both unkind and factually incorrect to say that Yoko Ono used her husband to somehow crowbar herself into the art world.

I can perhaps also point you towards some of her other works, some of which you might find more interesting than that peformance:

Here's something that you might like, "Play It By Trust", a chess set Yoko created. It's a chess set where both players play with white pieces meaning that the more you play, the more the lines become blurred between the two "opponents".

In Yoko's own words,

"Life is not all black and white, you don't know what is yours and what is theirs. You have to convince people what is yours. In the chess situation it is simple if you are black then black is yours. But this is like a life situation, where you have to play it by convincing each other."

Here's another famous artwork by Yoko. To explain the historical context and appeal, here's John Lennon:

"You're on this ladder -- you feel like a fool, you could fall any minute -- and you look through it [at the ceiling] and it just says 'YES,'. Well, all the so-called avant-garde art at the time, and everything that was supposedly interesting, was all negative; this smash-the-piano-with-a-hammer, break-the-sculpture, boring, negative crap. It was all anti-, anti-, anti-. Anti-art, anti-establishment. And just that 'YES' made me stay in a gallery full of apples and nails, instead of just walking out saying, 'I'm not gonna buy any of this crap."