It wasn't. The Post and ABC have asked this question about Trump six times, since September. At no point have more than 42 percent of registered voters said they think Trump is qualified. At no point have fewer than 56 percent said he is not qualified. That is a reverse of the numbers for Hillary Clinton, who has always been seen as qualified by at least 56 percent of voters.
Three-quarters of Republicans think Trump is qualified for the presidency in the new Post-ABC poll; 23 percent do not. By contrast, only 7 percent of Democrats think Clinton is not qualified, and more than 9 in 10 say she is.
This matters in part because there's (obviously) a correlation between the extent to which someone feels a candidate is qualified for the presidency and whether they will support that candidate. In other words, people who see Clinton as qualified mostly prefer Clinton's candidacy. People who see Trump as qualified mostly prefer his. Mind you, 11 percent of those who think Trump is not qualified for the presidency plan to vote for him anyway, suggesting either that they are in the group that views both candidates negatively or that they have a fairly loose definition of "qualified."
A key split in this race is on racial and educational lines, with the heart of Trump's support coming from working-class whites (particularly men). White voters without college degrees are the only group in which a majority sees Trump as qualified for the presidency.
That group is also the only one in which a majority doesn't think Clinton is qualified.
To some extent, people are using the word "qualified" as a substitution for "good candidate." But unlike numbers on the candidates' honesty and trustworthiness, there's a substantial split between how the candidates are viewed, including by members of their own parties. It hasn't impeded Trump, to this point; he trails Clinton, but by only five points in the latest Post-ABC poll. Among registered voters, his support in a four-way race has never exceeded the percentage of people who think he is unqualified. Luckily for him, neither has Clinton exceeded hers.