Taking the story at face value, I'd say she was unlucky. She was done for sending a "malicious electronic message". I assume that refers to Section 1 Malicious Communications Act, 1988:
http://www.cps.gov.uk/legal/a_to_c/communications_offences/#an13
The Malicious Communications Act 1988 section 1, see Stones 8.20830, deals with the sending to another of any article which is indecent or grossly offensive, or which conveys a threat, or which is false, provided there is an intent to cause distress or anxiety to the recipient. The offence covers letters, writing of all descriptions, electronic communications, photographs and other images in a material form, tape recordings, films and video recordings. Poison-pen letters are usually covered.
Just tweeting her message generally, rather than tweeting it at or about someone in particular, would hardly seem to meet these criteria. Still, if her prosecution makes people think a bit more about what they're tweeting, it might help to make Twitter a better place.