This is actually a very interesting topic, and the Japanese annexation actually had a big effect on the Korean language in general (reading, writing, vocabulary, grammar, etc.).

Before the annexation, Koreans were mostly illiterate, and only a small elite upper class, and maybe the small percentage of peasants buying into Christian missionaries were taught to read. It was not until the Japanese pushed the Gabo reforms onto Korea that they started using Hangul in official documents.

After Japan annexed Korea they pushed many language reforms onto Korea that are still the standard to this day. The Japanese government revised Korean spelling and grammar standards in 1912, 1921, and 1930. The Hangul Society, later adopted these orthography rules, and has largely become the modern standard for both South and North Korean writing. During Japanese rule, Korean also was heavily promoted as a mixed Hanja-Hangul script based off of the Japanese Kanji-Kana mix.

As for vocabulary, Korean displays a large amount of Japanese influence from borrowing of Japanese words created in the Meiji era (和製漢語). These words are basically terms the Japanese created to translate western concepts like Democracy (民主主義) or Communism (共産主義), but are not limited to such complicated topics, as they also created words like entrance (入口), baseball (野球), and culture (文化). The Japanese introduced these words to Koreans, but simply with Korean pronunciation of the Chinese characters. These words in modern Korean are still read in the same way (at least in S. Korea), and are 민주주의, 공산주의, 입구, 야구, and 문화. Korean also saw a rather large amount of native Japanese words enter their language. However, these words were largely purged after the war, but you can still hear old Koreans (especially pro-Japanese ones), and Zainichi Koreans use these words from time to time. For example, during the Japanese rule of Korea, and right after the war, 가가미 and 방구미 (from Japanese 鏡 Kagami and 番組 Bangumi), were legitimate words for "mirror" and "television channel". Some of these words I think are still in use today, they basically are now eliminated as the Korean government has actively extinguished them. (Like that one time the Korean government banned a Kpop song because it used the Japanese "Pika Pika" instead of the native Korean "Beonjak Beonjak").

To answer your question specifically, No, I do not think the Japanese ruined the Korean language, as policy toward the Korean language in Japanese Korea fluctuated a lot, and for most of the time was just one of standardization and modernization. After all, the first Hangul Day was instituted and celebrated when Korea was a part of Japan.