Southwest, which is the business model for most low-fare operations due to its fabulous success and ability to stay in business and makes profits year after year, is a dedicated 737 operator, and due to Southwest's success with the 737, most other low-fare airlines have followed in their path. Ryanair, for example, Europe's leading low-fare airline, went with the 737, as did others.
But many other low-fare airlines have had success with other types, AirTran operates the DC9S and is converting to an all 717 fleet, Jet Blue is using the A320 very successfully for its low-fare operation, in the past, PSA operated in California with first a 72S fleet and then coverted to the MD80 (which went to US Air and are now parked in the desert). A well run low-fare operation can work with any two-person cockpit crew, two engine, one-class layout, "smaller" airliner, including the MD80s.
Many of the low-fare carriers look to the 737 since a huge number of type was built in each generation and they were "easier" to acquire on the 2nd hand market. Also, leasing companies purchased the 737 is great numbers which increases the type's availability; the MD80 was not as popular with the leasing companies. Also, although many, many MD80s were built, a huge percentage of that type is in service with AA (directly and now through the TWA merger), Alitalia, SAS and Delta and simply never recached the 2nd hand market........when Swissair, for example, retired its MD80 fleet in favor of the A32X series, their aircraft were quickly grabbed by other airlines. Only recently have a large number of MD80s become available, mainly due to the events of 9/11 and the fact that the oldest of the type is a bit more than 20 years old. An exception: NW parked their small MD80 fleet (ex Republic I think) a while ago in Mojhave, and never found a purchaser......odd.
If MD80s are available at good lease rates or purchase prices, I think that a low-fare airline would do fine with them, provided, of course, that the other elements of the business plan were in order. Good luck to JetsGO.