The opposite of aesthetically pleasing is ugly since aesthetically pleasing means beautiful.
So the opposite of beautiful would be ugly or an eyesore.
Beautiful is another word for aesthetically pleasing.
Some other words for aesthetically pleasing are attractive, gorgeous, exquisite, charming, pretty, elegant, handsome, ravishing and Beau.
When something is aesthetically pleasing it's beautiful and great to look at.
The saying of aesthetically pleasing is another way to say something is beautiful and great to look at or enjoyable to look at.
For example a natural waterfall on a mountain would be aesthetically pleasing to most anyone in the world who saw it.