The compilers of the OED point out that they merely record the description of how the word is used, rather than prescribe how it should be used. The term, McJob, has been in usage at least 20 years, they say. It was popularised in 1991 in Douglas Coupland's novel Generation X: Tales for an Accelerated Culture as one of the margin definitions. Here it was described as "a low-pay, low-prestige, low-dignity, low benefit, no-future job in the service sector. Frequently considered a satisfying career choice by people who have never held one."
A "McJob" is a low-paid job.