Historically there have been two times in the last 15 or so years when shorting a basket of MLPs made sense. First was when the credit markets went to hell. The other was when interest rates spiked in the early 2000s.
Indeed as has already been said, for every position there is a counter position. There are probably more ETFs than stocks. Shorting MLPs would make sense in the short run if either credit became unobtainable as in 2008 or if a huge spike in interest rates made savings accounts attractive compared to MLPs. Nothing to worry about right now.