In german after the subject, verb always comes first therefor mag ('likes') will come before nicht, overall layout would be subject verb and negative.
'She likes the dress not' is what you need to rephrase the english sentance to before translating it to german.
I think, because of this (quoted from DL's tips and notes):
...Finally, "nicht" also tends to come at the end of sentences (after direct objects like "mir" = "me,"...
I think in this sentence "die Kleider" is a direct object