If you thought she got dirty in the main universe, where the log she was hiding in was evidently full of dirty water, it's nothing to how dirty she got in this alternate universe in which the log was full of mud. Fluttershy was completely covered in mud by the time she got out of the log and her dangerous mission outfit was beyond ruined, but she was far less worried about that than both Trixie's threats and the Everfree Forest.