In theory such a tool could be useful, but I would be worried about the usage restrictions imposed by the various sites. For example, many if not most freelance marketplaces are particularly concerned with any significant information about the jobs being made available outside of the official sites. Also, jobs can be posted as semi-public where they can be viewed only when logged in. To remain legal, it would be necessary to consult carefully each source site's EULA.
It would be very important to give a median score rather than a mean score. A resilient range, such as from quartile 1 (Q1) to quartile 3 (Q3) could be useful as well to mitigate outliers.
Another curiosity would be a map showing a colour gradient for the scores. I could see such a map being in popular reference.