Snopes fact-checks this quote (which has also been applied to Obama and Bush prior to Trump). The quote was actually concerned about good men running for office and being unable to connect with the masses that vote for them (this was before radio, television, the internet - when the only way for a candidate to get direct word out was to travel around the country for in person speeches). A fuller context for the quote:

The larger the mob, the harder the test. In small areas, before small electorates, a first-rate man occasionally fights his way through, carrying even the mob with him by force of his personality. But when the field is nationwide, and the fight must be waged chiefly at second and third hand, and the force of personality cannot so readily make itself felt, then all the odds are on the man who is, intrinsically, the most devious and mediocre — the man who can most easily adeptly disperse the notion that his mind is a virtual vacuum.

The Presidency tends, year by year, to go to such men. As democracy is perfected, the office represents, more and more closely, the inner soul of the people. We move toward a lofty ideal. On some great and glorious day the plain folks of the land will reach their heart’s desire at last, and the White House will be adorned by a downright moron.