What does 相比起来则 in this sentence?
You need a complementary sentence to understand this better.

Let’s say “Daniel is very fierce.”


  丹尼尔很凶。


With the context that Daniel is fierce, you can then compare Mary’s temperament using the grammatical structure you have shown.

相比起来则 comprises 相比起来 + 则.

相比起来 means “when you compare (Mary and Daniel)”

则 is a word which performs the function of “then, thus, therefore”.

You can then read the sentence as “Compared to Daniel, Mary is much milder.”