Black dress shirt at a funeral =
Black shirts are fine for streetwear and more casual looks (ones that do not involve ties), though I do think that a dark navy probably would work better in a lot of cases. Mariota looks terrible because he's trying to wear the shirt in a formal context with an absolutely awful tie. He's probably look better with a button undone and no tie, but still wouldn't look great.
All that said, the guy just won the biggest award in college sports and he's a young kid. He has plenty of time and future earnings to use to up his game in the clothing department if he is so inclined and can probably get away with being badly dressed in any event.