Entry from OED Online

[Either < MING n.³ + -ER¹ or < ming- (in MINGING a.) + -ER¹.] 

    An ugly or unattractive person, esp. a woman. Also in extended use.
 
  1995 Mixmag May 144/2 Jade..seeks like minded love warrier for spanking nights out, mingers need not apply. 1997 Guardian (Nexis) 13 Mar. 6 When I challenged people about what he had said they told me I must be mistaken and that he had used the word ‘minger’, which they said was a local word for a scruffy person. 1999 Bath Chron. (Nexis) 14 Apr. 17 Last night Ali asked fashion designer Thomas Del Jeffers if he was happy to design clothes for women ‘even if they were mingers, you know, nice personalities, but mingers’. 2000 Times (Electronic ed.) 2 Sept., Sing When You're Winning..is a catch-all record... In its aim to appeal to everyone it manages to alienate only those with hearts. It really is a humdinger of a minger.