Not sure what evidence there is that the quality of leaders has changed, but there is evidence that both (a) we perceive strangers differently (compared to say

Not sure what evidence there is that the quality of leaders has changed, but there is evidence that both (a) we perceive strangers differently (compared to say the 1960s), see Robert Putnam's book Our Kids and (b) there are cognitive biases that push us to think things are getting worse (declinism, rosy retrospection, positivity effect).