I honestly don’t think editorial identity has anything to do with why they’re not bringing in more ad money. Far as I can tell, tumblr is largely focussing on big brands and the process sounds far from frictionless. The widely-cited minimum buy number over the past couple years is $25k, and it sounds like targeting options are limited.
Meanwhile, you can reach everybody in the world on facebook with a few mouse clicks, and slice and dice the audience down with incredible precision. The process is near-frictionless and still allows you to reach a huge audience. That is the current gold standard.
Advertisers care far more about targeting the audience they want to reach than they do a cohesive editorial/publishing environment (within certain broad parameters). See the success of Outbrain, etc. Those units appear on a wide range of different kinds of publisher sites, the secret sauce is good targeting and low-friction ad buys.
Yahoo should put every resource they have into making buying as frictionless as possible and targeting as kick-ass as possible. There’s still plenty of room for innovation in this space.