Is my Xbox One temperature normal?

First of all, don't worry; 50 degrees is perfectly normal.

For future reference, the temperature of the air surrounding your console's main outlet vent is not necessarily reflective of the temperature of the CPU or graphics card(the primary generators of heat in a game console). As far as I know, there are no diagnostic tools to show you more precise information on an Xbox One, but modern CPUs and graphics cards will usually reach temperatures of 70-80 degrees under a reasonable amount of load without damaging the hardware.

The other thing to note is that the top vent of and Xbox One is the outlet vent. There are also 3 inlet vents on the sides and rear panel. If any or all of these vents are blocked by anything, be it paper or the cases of video games, the whole system may be experiencing sub-optimal air flow, which would reduce the effectiveness of your Xbox's cooling system.

Also,the CPU and GPU both have embedded electronic thermometers which are constantly monitored by the Xbox itself. If temperatures ever get abnormally high on the actual hardware components, the Xbox will give you a warning with ample time to clear the air vents or turn off the console. If they ever get dangerously high, it will turn itself off immediately(I would estimate that dangerously high is greater than 80 degrees).

Happy gaming ;)