Not a bad place, and certainly a good place to eat if you're staying at the guesthouse (as I was), but I'd say that for a vegan, the menu has some options, but it's very basic. Nothing really that you won't find or at least be able to request in any other Thai restaurant, so I'm not really sure what justification Mut Mee has for being on Happy Cow.
Still, it's comfortable, and a very nice place to sit right on the banks of the Mekong. It's surrounded by trees and plant life, which although pleasant, meant that when I was sitting here in the evenings I was getting feasted on by mosquitoes, and I'm someone that doesn't get bitten often, so be prepared for that. There is no indoor seating area.
Other than that though it's a nice place with basic vegan choices. And considering what little else is available in Nong Khai, you could certainly do worse.