Sri Lanka is facing difficulties but I don't understand who the "we" is in this question. Is it the United States? If that is the case my only answer is absolutely not. Sri Lanka is small but there's no need for super powers to hone in and turn it into some nation it doesn't want to be.
While it is important that world leaders work together to help keep peace worldwide, stepping in and taking charge of situation constantly only leads to further destabilization further down the line when the time comes to leave. World with these countries externally and help them find ways to prevent violence on their own, and let yourself room to deal with internal problems.
No, we should not undertake nation building in Sri Lanka, because the mob violence there was an isolated incident. There is no evidence of any kind of ongoing struggle or culture in the country on a large-scale or long-term basis. We are spread too thin as it is and we cannot handle any more overseas commitments.
No, this has been the problem in Iraq. We do not understand the cultural dynamics between Sunni, Shia and Kurd, so we installed a Shia dictator who marginalized the other cultural groups, creating the myriad of problems we have in the country today. If we attempt nation building in Sri Lanka, it is likely to end in similar fashion because we just do not understand the dynamics involved.