I'm not sure you do, finishing 4th out of 30 doesn't meet the definition.
me·di·o·cre
ˌmēdēˈōkər/
adjective
adjective: mediocre
of only moderate quality; not very good.
"a mediocre actor"
synonyms:
ordinary, average, middling, middle-of-the-road, uninspired, undistinguished, indifferent, unexceptional, unexciting, unremarkable, run-of-the-mill, pedestrian, prosaic, lackluster, forgettable, amateur, amateurish;