The M suburbs (Mooroobool, Manunda and Manoora) have a (not always deserved) bad reputation and cetainly the crime rate is probably higher there than a number of other suburbs. A good guide is the SEIFA Disadvantage, where suburbs lower on the list have greater disadvantage (http://profile.id.com.au/cairns/seifa-disadvantage?es=2.). Mooroobool is affected by light aircraft and helicopter noise, but not jet aircraft which tend to affect Cairns city, Parramatta Park, Cairns North, Areoglen, Machans Beach, Holloways Beach, and Yorkeys Knob.
Mooroobool has a bad reputation and when we got here people warned us not to buy in a 'M' suburb. We did anyway. It's close to everything, our street is wide and the neighbours are friendly. Mooroobool is a fairly big suburb with some parts worse than others. I have lived in City View, above or west of Irene Street, which some call Upper Mooroobool, for 18 years and have had no problems whatsoever. I wouldn't live anywhere else in Cairns.
I wouldn't touch the city or the surrounding suburbs. Crime is concentrated in the M suburbs including Parramatta Park and Bungalow. South is a ghetto IMO. Anything Trinity Beach and north of there are the best choice.