Erna's Elderberry House is in the area http://www.elderberryhouse.com/
Never eaten there, but it has the reputation of being one of the best restaurants in the area. Theres also the Ahwahnee and The Mountain Room in the actual park. Each are rather to very pricey options. There is a good collection of restaurants in Oakhurst as well where the 41 freeway and 49 intersect in town. It's a good little area to explore. And theres Gus' Steakhouse in Sonora off the 108, where they serve a nice complimentary fondue before dinner.
Hope that helps a bit.
Jantz is worth going to on your way to or from Yosemite (From the bay area), but it is a bit of a haul from Mariposa, and after doing the haul from Yosemite to Mariposa, you won't feel like going another hour plus to Jantz....
Best restaurant in Mariposa is Savourys, worth trying for both breakfast and dinner...
If you are going to be there more than a day or two, consider heading to Wawona near th Park's south entrance, via 49 to 41 through Oakhurst. (ignore googlemaps directions to take Chowchila Mtn Rd, which you definitely do NOT want to do). I find it a nicer, less stressful drive than from Mariposa to Yosemite Valley, and the Big Trees in Wawona are definitely worth a visit. If you do so, you could try dinner at El Cid in Oakhurst on the way back. I haven't been, but it has gotten some good mentions on CH, and when I drove by the other day I noticed that the parking lot was quite full, even early on a weekday.
I have also had a nice meal at Sal's in Mariposa, though they don't have beer, which is hard to take after a hike...
(wanted to add some links, but can't get them to work right now)