That's actually not true! And I can explain why :]
At the core of what a reader wants from a character: they want them to change and learn something by the end of their story. Their personality (or how likable they are) is irrelevant to that process!
This means that often the more screwed up the character, the more we actually like them. Because they have a lot of room to develop and become better. Their story can go into different directions, depending on the facets of the character that we want to tackle. They're ripe for stories.
This is why we often like characters that are screw-ups, the under-dog, the average loser, and jerks (well-meaning jerks especially break people's hearts). We can like nice characters who are good at what they do and have pleasant personalities, but if they don't have a flaw in some way (usually the flaw is naivety, being "too nice", over-working, etc) to develop out of, the reader won't like them. Some poorly written characters that are just "nice" for the purpose of being "likable" actually starts to creep people out.