Bayern controlled the clash with their relegation-threatened visitors, and a spectacular overhead kick from Franck Ribery in the 20th minute was enough to seal the victory.
Frankfurt goalkeeper Lukas Hradecky made saves from Juan Bernat, Xabi Alonso - who also hit the crossbar with a free-kick - and Thomas Muller in the second half, but Pep Guardiola's men were unable to add to their tally.
READ MORE | Ribery stunner enough for Pep's men
Lewandowski conceded that Bayern's players were already thinking about taking on Benfica in the Champions League quarter-finals in midweek, and that may have come at the expense of entertainment.
"It was not an interesting game for the spectators, but the three points were important," said the Poland striker.
"We certainly had the game against Benfica in the Champions League on Tuesday in our heads.
"We have to play much, much better than we did today [against Benfica]."
However, Guardiola saw plenty of positives in the result, reserving particular praise for Ribery.
"We started well and had a lot of good actions - not only the wonder goal from Franck," he said