Bad personality is one of those vague terms that almost doesn't mean anything.
Someone will describe you as having a "bad personality" if they just don't like you, no matter what the reason is. It will mean different things to different people. For some people it will mean they don't like the way you look, for others that they don't like your political beliefs, for others that they don't like your sense of humour, etc.
It's just like Harvard rated Asian and White students as having a bad personality/character compared to Blacks and Latinos, in order to justify affirmative action. Personality/character was vague enough so they could tweak the numbers however they wanted. Thankfully, the US supreme court put an end to that bullshit.