"Mother is my life" means your mother. It is grammatically correct but it is not contextually or culturally correct for Mother's Day or as a form of praise.
If you are trying to thank your mother, it is not a good idea to make comments that sound as if you are talking to someone about your mother, which is what your sentence sounds like.
If it's Mother's Day, you want to address her directly. And that phrase is not right in terms of direct address. Of all the ways of saying your mother has played an important role in getting you to a certain point in your life or how important she has been, that is not the phrase to use.
It is very, very weird in the English-speaking world to say that. Over-attachment to mothers is a serious psychological condition.
The movie Psycho by Hitchcock is an extreme example of this phenomenon.
I do not want to write the card for you to your mother, but I suggest you find another sentence. However, the sentence might sound more like: "Mother, you are a very important person for me." In any case, I would use direct address and not indirect address.
Many times one sees these endearments starting with thanking a mother:
"Thanks for being such a great Mother!".
"Thanks for being my mother!".
"Mother, you're the best!".