It would laugh at it too. It just doesn't work. I think there are several reasons why it is so funny. First, it is 敬体 (Keitai), it would probably umimaginable for any Japanese to pick a formal sentence to be inked in your body. I'm like "To whom are you being polite??" Second, the word "頑張る" is not an appropriate word for a tattoo. it is too bright, too healthy, and I would imagine that people who really 頑張る don't need a tattoo to represent anything about themselves. It would be wiser to choose a noun which represents a "concept" with the element of 頑張る. Wouldn't you find hysterical to see a tat that reads "I will do my best, sir."???

It would make me laugh ^^. It's not cool so much X(. I want to ask him/her ”what's your goal"?

I may say him "おつかれさま" (^_-)-☆

頑張っていますね！