Do you two already use the word "love" in your relationship?
If you do I think it is a sweet gesture. Everyone likes to hear positive things.
If you don't use that word in your relationship be prepared for it to be awkward. Of course you could always change ur to "reasons why you are the best T" or something.
As an aside how do you know your T's birthday? I have been seeing my T for 5 years and we are very open but o do not know her birthday. Of course I've never asked bc I felt giving a gift or card might make it seem like i expect something In return and that wpuld be weird