Opinions about origins of loljobs vary. Loljobs are different from lolcats
Most common job to coin this phrase was DRG. It gained lolDRG moniker when for years Square refused to upgrade Call Wyvern from being 2hr ability back when grips didn't exist and dual-wielding had nearly twice as better TP gain. Back when "looking for group" option was used on daily basis to compile parties DRG was least picked choice for melee slot. Spending MP on wyvern's master after Spirit Link or letting them have time to rest was deemed unnecessary and another inferior point compared to other available melees. Even if player knew what they were doing and had good gear on them lolDRG moniker was giving bad reputation to entire class.
Some argue that original loljob was MNK when ability to launch Asuran Fists with Relic Knuckles nearly back-to-back was reduced to its current more moderate form. Thus loljob may refer to a job that has been nerfed in some way. Previous candidates have included lolRNG after RNG major nerf which tilted RNG from top melee spot it held for years and lolBLM, also after a nerf that brought cumulative resistance to HNM.
BLU demonstrates different crowning of the name. Since its release BLU had difficulties to adapt to end game scene as its abilities didn't do remarkably better and it required a lot more gear sets and outside maintenance due lack of native refresh spell. People who tried this and were overwhelmed or knew impending limitations of BLU started to refer it as lolBLU.
Even after Square's continuous efforts to balance out some major job differences loljobs are called out now and then. It's important to remember that shit is situational and there's love for everyone.