Here are some of the guesses and thoughts from me and others.
Tracer is because she blinks like a trace round.
Widowmaker is just the spider thematic also the part where she made herself a widow by killing her husband.
Reaper was given to Reyes because people say that he looks like the grim reaper.
Some names like Pharah and Zarya are nicknames to pronounce their names easier, fareeha amari and alexandra zaryanova, respectively.