"Utsukushii" and "kirei" are synonyms.
I looked for an appropriate expression in a thesaurus.
類義語 refers to words that are merely similar in meaning, whereas シノニム (like 同義語) implies an essentially identical meaning.
thesaurus
Compound of 類 (kind, sort) + 義 (meaning) + 語 (word). A straightforward kanji-based term for 'similar-meaning word'.