Not sure about that (haven't used yarn however), YARN seems extremely Hadoop specific, whereas Mesos is a "closer to the metal" generic framework to build frameworks for scheduling things on.
If I was to compare the two, I'd call YARN an application scheduler and Mesos a more meta scheduler. You could build yarn ontop of Mesos. You likely couldn't easily build Mesos ontop of Yarn. It should be relatively trivial to make a YARN Framework, which someone has already done with Mesos:
True but Hadoop is the whole tooling and with YARN they are distancing themselves from Map/Reduce. IMHO, it looks like they are heading in that right direction but not quite there. That why it maybe v4 before we can think of Hadoop as an OS for datacenter.
Mesos is closer to the metal but still is only a piece of what is needed. Picturing it as an OS is misleading while picturing Hadoop V2 as a proto-OS is not that much in accurate.
YARN is still in its infancy and doesn't run in production for user-facing applications at any large site. Also, there are big limitations when it comes to scaling YARN. YARN also has a lot of overhead on each machine the worker processes run on.
Twitter runs mesos in a multi 10k node cluster for user facing, production applications.
https://www.quora.com/How-does-YARN-compare-to-Mesos