Granted, if you're doing something longer that gives you more time to screw up, but no, second terms aren't unlucky. If anything they give the president more power, no longer having to be concerned with getting elected a second time. Obama has shown this already by appointing who he's wanted to regardless of objection.
The second term is the better term, in general, as the president no longer needs to worry about reelection and can press harder in order to get what they want passed. This can cause people to be annoyed and frustrated with them as they go for the more extreme side of their running platform.
"Unlucky" is a strange term. Usually in the second term presidents are less careful than they were in their first--they are willing to do things and take on problems that would have ensured they didn't get elected to a second term if they opted to work on those things during the first term. I just think they are more forceful during their second terms, which can also cause more people to fight back against them.
I do not feel that second presidential terms are unlucky. It seems that often times when a president is elected for a second term they get to deal with many more of the issues that are important to them rather than the past four years where they often times are cleaning up the past presidents mess. I think the terms are not unlucky i think it is the president and the choices they make that can cause issues.