What is a bad Spam Score?
Moz uses 17 flags (number of factors that may or may not increase spam score) to measure the spam score. Below is the ratio between the number of flags and the probability of spam score.

This is how Moz categorize a bad or good spam score.

Spam Score Level
1-30% Low
31-60% Medium
61-100% High
Why do high domain authority and lower spam score matter?
Keeping higher domain authority and lower spam score helps to focus on ranking factors including:

Building quality links
Avoiding spammy links
Improving user experience
Enhancing social media presence