I'm looking to create a report that gives us a 30 day trailing average if lead scores are increasing or decreasing any ideas?

You could just pull a smart list of "lead score was changed" in the past 30 days, then sort by the lead score. That's probably the easiest. 

You could try doing this in SFDC with a report that shows scores.

You might also try Leads by Score report in Past 30 Days with a smart column of score.

The best way may be to have SFDC run that report with Avg and then tally that in a sheet each month.

Salesforce will be the best place to run reporting like this. As Josh outlines, you will want to create a summary report that gives you an average of Lead Scores and export it each month.

Alternatively, there is something called analytic snapshots that you can setup in Saleforce. This allows you to run reports and save the report results as records on custom objects. These can then be used to track trends within SFDC, no exports needed. You should ask your admin to help you set this up.