Her English has gotten better than it was last year.
Her English has improved a lot since last year.
この場合、次のような言い方ができますよ。
ーHer English has gotten better than it was last year.
「彼女の英語は去年より上手になった」
to get better で「良くなる・上手になる」
ーHer English has improved a lot since last year.
「彼女の英語は去年からかなり上達した」
to improve で「上達する」
ご参考まで!