First off, welcome to ME.
Your car is at best a 16 second car in the quarter mile. More than likely a 16.5-17.0 @ around 80 MPH anything near that would be acceptable. The fact that you are running the 1320 in 18 seconds could come down a combination of multiple things, seeing as it's a 5 spd, driver is always a variable (don't care how good you are, you will never be as consistent as an automatic), tires, track, atmospheric conditions, mechanical issues, weight of your car, hardtop or convertible, big subs in trunk, etc..
What is your trap speed? I'm assuming around 82 ±5 MPH, if so, your car is fine, again depending on conditions. If you are running that speed at altitude like in Denver or in otherwise horrible DA (density altitude), then you and your car are fine. If you are doing this near sea level with nice cool air (lower DA), then more things come into play to explain the slower than expected time.
One thing you need to realize, you just simply have a slow car. Not trying to piss in your Wheaties but all V6 Mustangs prior to the 2011 and it's 3.7 were turds.
Without knowing more things about your car, it's condition, your ability as a driver, track conditions, DA, car weight, etc.. It seems like your car could be performing within reasonable standards. Just simply make sure your car is running correctly, learn to get the best out of it and if you feel like wasting money instead of saving for a car that actually has potential, go with a tune, rear gears and good tires, otherwise, drive the car, accept it for what it is and save up for something that has potential to actually be fast.
99 Chevrolet Camaro Z28 M6 - 6.676 @ 103 in 1/8, 10.512 @ 130.2 on street tires, H/C/I
93 Eagle Talon TSI AWD 5spd - Built 6 Bolt, 16G Evo3, HKS 272 Cams - under construction
2002 Pontiac GTP 2 Door (Daily) - Gen V swap 3.2", headers, FMIC, rockers, 14 PSI - 13.303 @ 108.11