Does Tumblr have any place in the advertising plan of a DMO? Short answer: probably not. Although I'm sure there is quite an inventory of different ages, Tumblr's audience is very young. Traditionally, in the DMO space, the young are not considered travel decision makers or influencers. Although, there are going to be exceptions to the rule, Tumblr is a very visual and quick medium and Tumblr ads would likely function very similar to display in terms of branding with less direct action. I have seen some surprising brands advertising on Tumblr. A popular manufacturer of light bulbs, for example, had animated gifs of abstract lighting and effects running with plenty of likes and interaction. It doesn't seem like Tumblr ad content has any less interaction than average user-generated posts, and in fact seem to get more interaction than I personally would have expected.
This can be valuable, but I have not worked with many DMOs that would be comfortable having their brand potentially next to sensitive user-generated content. Yahoo and Tumblr may have filters or ways to pacify queasy advertisers, but my gut says if your brand isn't ready for that, then it is probably not a venue for you. Perhaps more youthful, "edgier" travel destinations like Las Vegas, New York City, etc. may be able to get mileage out of Tumblr but my experience with most DMOs tells me that Tumblr is just not the medium for them.