Despite taking the lead through Gareth Thomas' third-minute try, the French hit back with Thomas Castaignede, Vincent Clerc and Frederic Michalak all touching down as the visitors were out-classed in the Stade de France.
However, Williams said although Wales had earned the dubious honour of their first Six Nations whitewash since 1995, the team had to keep looking forward.
"Unfortunately they are a very good side and they kept ball and kept us down in our 22," he said.
"I think we made a lot of mistakes, missed line-outs and missed touch kicks, but they deserved the win."
Williams denied that failing to win in this year's Six Nations would increase the pressure on coach Steve Hansen ahead of this autumn's World Cup in Australia.
"We are very disappointed. We have put in a couple of good performances against Ireland and England and with some patience with this squad we can do well at the World Cup," he added.
"We have showed we can compete with the best and it (Stade de France) is a very tough place to come and get a result.
"I think the coach has done a great job and we need a bit of patience in Wales to see us through," he told BBC1.