Does the MAX31855 thermocouple converter output accurate temperature values?
Thermocouples are approximately linear (a few percent for many popular types) so it makes sense to specify the resolution in units of temperature. 

Yes, you have to apply a lookup table, polynomial or other linearizing function if you want accurate temperature readings over a wide range. If you only care about some particular temperature a linear scaling may suffice. 

Also make sure you have the correct type of thermocouple attached. The difference between J (Iron-Constantan) and K (Chromel-Alumel), for example, is about 20% of temperature difference from cold-junction. The cold junction compensation of this chip is a crude linear approximation of the temperature output of the thermocouple near room temperature, so it also won't work properly in general if you attach the wrong thermocouple type (though some are very close to each other such as T and K). 

You don't specify the type of T/C but I suspect you may have a J T/C connected to a K chip. J thermocouples of North American origin generally have black color code.