To be honest, I don't mind it. You have to pick your battles, and there's far more important ones for them to fight on behalf of animals, so it's a bit silly, but at least the sentiment is there. They didn't say it offends cows - that's obviously media spin designed to ridicule - they said it 'mocks the misery of animals.' And, in fairness, it was only a tweet.
Bullfighting is possibly one of the most evil things to exist in this world, so if it raises awareness and gets people to pay attention to the reality of it then that's great. Maybe one day it will be banned everywhere.
On a similar note, when I hear boxers and commentators make metaphorical use of dogfighting terminology, I want to kill them. The two should never be conflated.