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Last tested: Aug 30, 2018
According to the doc, positive numbers will display an upward arrow and negative numbers will show a down arrow. You can flip this so that "Positive values are bad".
Beyond that, you'll want to use table calculations to manipulate your values accordingly.
For some use cases, we might want to compare two values, for example: Sales value this month vs last month, we want to use arrow to indicate if this is an increase or not, we can use a table calc to do this using if() function.