“めっちゃ” is a dialogue in Osaka district and is often used by young people. It means "very " and is equal to “とても” in standard Japanese. So the example above means “This noodle is very nice！” I would like to add one more explanation. The ending of the sentence is not "じゃん" but "やん" in Osaka district. So it is expressed like "ラーメンめっちゃうまい"や"ん！" in Osaka. "じゃん" is a dialogue in Tokyo district and there "very" is expressed like "すげー". So it is expressed like "ラーメンすげーうまいじゃん！" in Tokyo. Please note that these sentences are usually used by young people. Hope this could help you. Thank you.

it means "very" or "so"

It is a dialect of じゃん Tokai misunderstood with a dialect of じゃんは Yokohama, but it is the dialect of the Tokai district

应该是めっちゃくちゃ的简略。它的通常字典的意思是乱七八糟的意思，但是经常被日本年轻人用做表示很，非常的意思，与之同义的年轻人用语还有ちょう。