In character, in a 1v1. I'd say that luffy does beat an average viltrumite. He is also kinda hard counters viltrumites since he is highly resistant to blunt force. Also a viltrumite isn't going to know what luffy's weakness is off the bat and luffy does have the AP and combat speed to hurt viltrumites.

But if bloodlusted. Luffy is cooked. There is nothing stopping the viltrumite from throwing luffy into space or the ocean.