Is it true that Ivy League schools' professors are the easiest graders?

Not in my experience. 

I've attended undergraduate college classes at an Ivy League university (from which I graduated with a bachelor's degree), another competitive private university, a "flagship" state university, and a community college, and the grading went from most difficult to easiest in exactly that order. 

At the community college, they "taught to the test." Exam questions were taken directly from the text and the homework, and the professors' lectures emphasized exactly what the class needed to learn.  

At the state university,  exam questions were taken directly from the text and the homework, but the professors didn't necessarily underline what you needed to learn in the lectures. 

At the private university, most of the exam questions were taken from the materials, but there were always one or two questions that made you apply your knowledge of the materials to novel problems and circumstances. 

At the Ivy League university, most of the exam questions assumed you had fully learned the materials and understood them; and nearly all of the questions required you to apply your knowledge of the materials to novel problems and circumstances. Only a few questions on an exam would be straightforward questions about the materials presented in the reading and lectures. 

As Sam Spayed answered, that was not my experience either. I went to an Ivy for a BS, a State University for an MBA, and a different State University for a MS. I found that the professors I had for my first three years of undergrad were pretty tough graders. As I got to my Senior year, it seemed to ease up a bit, and in grad school, it seemed relatively easy to get a B, much harder to get an A. 

I don't know how much of that is due to the nature of the school, vs. grad/undergrad, vs. differences from one professor to the next. 

I think it also matters whether you are in a quantitative vs. qualitative field. In a quantitative or science field, often the grades are based on non-subjective tests where your answer is either right or wrong, and your grade is what it is. In a qualitative field, it might be easier to get a passing grade, but harder to get an "A" as it's more subjective. 

Ivies' reputations rely on tough programs with hard-earned grades.