Should I show this Dog?

If she is able to behave herself around other people and dogs and letting strangers examine her (like a judge at a show would) then I don't see why you shouldn't show her even with her background. But if she's not comfortable around as many dogs and people as it tends to be on a show and she'll react by growling/barking, then I think it would be far too stressful for her.

Hi I used to own a show dog and know a bit about dog shows. Seeing other English Mastiffs shown in contest, I's say your dog is just pet quality. The angle of the muzzle with the forehead is an eliminating fault in a dog show. Of course, you can ask for more opinions or even go show your dog, but the last option seems unnecessary to me; you would only lose money and time doing it. Your call. PS: you didn't post a full photo of your dog, in the standard show position. I might offer you more details if you do. Good luck and keep loving your dog, she is beautiful as she is.

Your dog is beautiful! Such soulful eyes! Go ahead, show her!