My 5 month old daughter has been in the NICU since she was born and has had several surgeries, etc. She's on SSI already, but is she considered diabled?

A disabled  child under the age of 13 is treated no differently on an  income tax return so it really is immaterial if you answer yes or no. 

What is important is that you get the SS# before you file the return  AND  that you indicate the child lived with you ALL year even if they never stepped foot in your home ... READ the screen ... it gives you the answer.  And the next question about support ... even with SSI the child did NOT support themselves.  

Do I get a credit or deduction if my child is disabled?

If your child is older than 12 and you need to pay someone to take care of him while you work, you can say he is disabled and you can still get the childcare credit on your federal return.   Or if he is 19 or older you may still be able to claim him as a qualifying child dependent on your federal return.

Aside from those things, there is no federal tax advantage to saying that your child is disabled.   Your state tax laws might be different.