average(neither very good nor very bad) adjective [UK: ˈæ.və.rɪdʒ] [US: ˈæ.və.rɪdʒ]
lackluster(Not exceptional; not worthy of special merit, attention, or interest) adjective [UK: ˈlæ.ˌklə.stə(r)] [US: ˈlæ.ˌklə.stər]Her results last year were lackluster. = Sus resultados el año pasado fueron mediocres.
mediocre(having no peculiar or outstanding features) adjective [UK: ˌmiː.dɪ.ˈəʊkə(r)] [US: ˌmiː.dɪˈo.ʊkə(r)]I don't want to live a mediocre life. = No quiero vivir una vida mediocre.