Definitely don't worry about it....
Some schools continue the Dean's Letter a "Letter of Recommendation" others consider it a "Performance Evaluation". Some deans feel obliged to put in something less than flattering so... and this really is a quote... "the letter is balanced, and the overwhelmingly good comments are believed". Whatever...
Seriously, don't sweat it... I would, however give you the following recommendations on handling this eval if it comes up in aninterview:
Don't give excuses for the evaluation.... if anything, uplay the significance of it and describe how the feedback on your "less then optimal performance" has helped you do better on your other rotations.
Just my opinion... but it always seems in your best interest to explain how a negative comment, grade, whatever... has helped you grow as a future physician than explaining why it isn't true (even if it ISNT true)