Niu Youyin's martial arts were not considered poor.
As previously mentioned, both he and his brother Niu Youjin possessed decent talents, and they were by no means lazy; in the years since their escape from Shaolin, though they hadn't meddled in the martial world, they never fell behind in their training.
The nicknames "Silver-footed King" and "Golden Foot King" were not self-proclaimed but rather given by those martial artists who, passing through Guofeng Col and too proud to pay the toll, ended up being defeated by the Niu brothers.
Of course, if you were to say Niu Youyin was extremely strong... he wasn't, but he could be considered a second-class expert.
But "second-class" was ultimately a general assessment; in a fight, there were many factors that could influence the outcome: age, experience, physical strength, Inner Strength, tactics, willpower, luck, and the proficiency of practiced martial arts... all could blur the boundaries of these so-called "class" distinctions.
