Arjan Shahani argues at Americas Quarterly that Mexico needs a presidential runoff, so that winning candidates have a stronger mandate than 35 percent of the votes, and some 20 percent of the total electorate. True enough! A second round would certainly alleviate that somewhat unseemly facet of Mexican politics, and would certainly an improvement. (It would also give journalists an easy topic to write about for another several months.)
However, that lack of mandate in and of itself isn't Mexico's foremost problem; the real issue is that weak presidents have a hard time enacting any agenda. But that's not so much a factor of the lack of support for the president as it is a product of a tripartite political landscape existing over top of a presidential system. If Calderón (who had approval ratings brushing up against 70 percent for much of his year, you may remember) had won a second round against AMLO, presumably the PRD would have emerged less intransigent, but he still would have had to deal with a majority-opposition Congress, one in which the incentives for the opposition were still to block or water down any presidential agenda item that comes down the pike. That's the problem that needs to be addressed, and I don't see any easy way to do so.