She was not that bad, given that her character was someone on the edge of the "establishment". I do not think that gave her much scope to do much - speech wise.
It is hard to say, though, as there is not much more than Downton to go on.
Having said that there was one scene where she was particularly weak. She was looking through the patio doors for her mother. There was no reply to her banging and shouting - silence - and the doors were locked. In a panic, quick, quick, she had to run round the house to try to get in through the back door.....
To say that the panic stricken run was a bit girly - six year-old running to granny for sweets - would not be unfair.