How old is Muffy? Is there any chance she was rude to the other dog? When you say "attack" what do you mean, is Muffy hurt? It's normal for an older dog to tell a rude puppy off and maybe put teeth and pin the pup down but pup should not be hurt. How adept are you at reading dog body language?
Where were you and the other owner when this happened and what were you doing? I see parks where the owners and dogs walk in, let the dogs off leash and then the owners stand around and pay no attention to their dog. Big parks with walking trails where everybody keeps moving are better, most dogs go with their owner so don't spend a lot of time with a strange dog.
Some dog parks have a section for small dogs that is separate from the big dogs. Big dogs can hurt little ones in rough play without it being intentional.
Does Muffy have a small doggy friend to go to the park with? Be careful though, even two dogs can be a pack and intimidate another single dog.
Can you go in off hours when fewer people are there?
Good luck. It's nice to be able to let dogs run off leash, much better exercise for them than leashed walks. But dog parks can be bad places. Some have rules posted, does this one? Have other people had trouble with the big dog? Personally I have only gone to dog parks when visiting my sister. One she no longer goes to herself is ridiculously small, two acres, I'd never in a million years put my dog in such a tiny place with other dogs I don't know.
Oh, one last thing. I have seen people take their dog on leash into an off leash park. Very bad idea. Leashes impede body language. It should be all dogs off leash or all dogs on leash, according to the park rules.