While we can hypothesise about the impact of Marijuana on the Hep C drugs I would estimate in excess of 20% of my patients used it while on treatment and their success rates were similar to non smokers.
Veterans Affairs in the USA have also found similarly (although > 50% had a substance abuse problem)
As you can see for the group with defined substance abuse the SVR rate was 95.5% and without substance abuse 96.5%. The p value of 0.580 says that difference is not statistically significant.
The VA defines substance abuse as illicit drug (i.e., marijuana, heroin, methamphetamines, cocaine, non-prescribed opioids, etc.) and alcohol.