Way too hard on himself; was a decent actor
I loved Victor Mature, but the one thing I didn't like is how self-deprecating he was. He was always joking about what a terrible actor he was and such, when that clearly wasn't true. Mature was also actually an above average actor, much better than Charlton Heston, who was always seen as the bigger star.
For instance, one of the reasons why The Robe is a better than average movie is his performance as Demetrius. He was so good that his character was given a sequel because he stole so many scenes from Richard Burton.