Are there any dictionaries that show the relative frequency of 多音字?
As far as I know, there is one dictionary for this: 《现代汉语频率词典》，1986，北京语言学院出版社.

However, this dictionary is a little old and difficult to find. What's more, some of the readings are outdated, because 《普通话异读词审音表》 was published after the dictionary gathered the data.

The Unihan database contains the kHanyuPinlu field which is from 《现代汉语频率词典》. For example:

和:

kHanyuPinlu     hé(9513) huo(38) he(24) hè(9)


乐

kHanyuPinlu     lè(283) yuè(54)