Generally it's too dry in Tunisia for mosquitos. What the little blighters like is warm and wet. Skanes and Monastir are fairly low lying areas and there are areas where the sea encroaches on to the land, so warm and wet. this is probably why Skanes has the reputation for being the mosquito capital. It also explains the bad smell you sometimes find in that area.
Mosquitos love me. I can go to Sunderland and get bitten. Scotland in August is a total no-no. In Salou, Espana I was unable to walk for 7 days due to a bad reaction.
Tunisia - beeen loads of times- often to Monastir. Been bitten once in May and twice in August. No reaction, it's not really a problem. If you're susceptible to the s*** take reasonable precautions especially at night. Try citronella if you don't want a spray. I will be very suprised if they are bad enough to spoil your holiday. Inchallah