That depends a lot on how you're defining negative advertising. If you're in charge, that's up to you. If your boss has a say in this I'd say don't allow it, it's too much of a grey area, not worth risking getting yourself in hot water over. From a purely definitional standpoint, it's advertising, it's political, and it's negative/critical. Failure to mention the incumbent by name doesn't change any of that.
From my personal perspective, if you're in in charge, it's ok. I don't even think not mentioning the incumbent by name is important here. In general, if I'm in charge of a policy like this, what I really want to avoid isn't negative advertising, per se. It's mudslinging. This ad contains nothing personal or irrelevant to the election. It contains only verifiable statistical facts. What I'd prefer to see, personally, is an actual number for property taxes in 2005 and at present, but even so this would pass the smell test for me.