MSNBC is taking things out of context by not showing the full exchange.
Go to 41:20 to see the first question from that reporter. And go a couple minutes back if you want the larger context about the government acquiring a large amount of a medication that could potentially prove effective against COVID-19 because it reportedly was effective against SARS.
To which the reporter responded:
"Is it possible that your impulse to put a positive spin may be giving Americans a false sense of hope..."
When Trump called him out for a nasty question, I think he was referring to not merely the question just asked of him, but the one which immediately preceded it as well.
The tone of the second question seems to carry with it the suggestion that he's doesn't think the President's answer to his first question was good enough as a message to the American people. You could take his second question as challenging Trump to tell the American people something different - something not so positive. And if that's the case, then Trump was right to call the reporter out for being a terrible example of his profession. What kind of reporter would insist at a time like this that the President should be more negative during a crisis and not let people have any hope?