Ok, so I understand that "nosi" is translated as wears (present simple -PS) rather than "is wearing" (present cont.) because there is a more appropriate way to express the statement "is wearing now". However, "wears an ugly shirt" implies that he always wears the same ugly shirt whereas if we're talking about a routine (PS) then it would more likely be "ugly shirts".
So it should either be "is wearing an ugly shirt" or "wears ugly shirts".
I didn't get the question wrong because I knew from experience what was expected but the question is a little inaccurate.
This course has too many sentences with general "nosić" (that translates to Present Simple) while mentioning a singular object instead of a plural one. That's not that great.
But well, it does mean "an ugly shirt". Maybe he just wears one ugly shirt relatively often. I don't think he wears it every day ;)