It could have different meanings like it is hard to understand her or it is hard to deal with her..

She is not an easy going person. Difficult to deal with (sometimes).

She is somewhat hard to get along with. Sometimes when I hear "a bit difficult", it is polite way of saying that the person is quite difficult to deal with. To make that point, I would say: "She is JUST a bit difficult." A native English speaker would recognise the irony. If I specifically mean that the person can be dealt on a normal basis, with for the most part, but sometimes gets hard to be around, I would say: "She is a bit difficult at times."

'She is a bit difficult' can mean that she is an introverted and overly sensitive person.

If someone is "a bit difficult," I would think the person being spoken of is probably not very friendly and rarely smiles. It wouldn't be easy to get along with that person. On the other hand, maybe it says something about the speaker, that the speaker is lazy and doesn't do her job and that's why the other person is so unhappy and "a bit difficult."