The Italian coach was visibly unhappy with his side's display against the capital club and admitted that the result was the only positive to take from the match.
Goals from Diego Milito and Giampaolo Pazzini allowed Inter to extend its run of wins in all competitions to eight after Tommaso Rocchi had given the visitors the lead on the half hour mark.
"The only thing I liked about today's match was the result," Ranieri told Sky Sport Italia. "Against Milan we had a good match, but today the result was the only good thing.
"I think we could not play worse than we did today, as the only thing that we showed was a great character. Lazio created a thousand problems for us and we won only because of two individual actions from two champions, but we must do better from now on if we want to catch the top clubs."
Wesley Sneijder came on as a second half substitute, and Ranieri explained that the Dutchman has not yet fully recovered from a muscular problem.
"Sneijder is not 100 percent, but he is recovering and becoming important for us again," Ranieri added. "Against Genoa [in the Coppa Italia] he played well and today I introduced him right after the break, because I did not like anything about the first half and I was hoping he would change something."