That's not a good idea, imo. If you want a secure provably fair system then the server seed has to be generated randomly using a good and reliable RNG. If you are going to hash something like "hello word" or anything like that then there will be a possibility someone with enough computing power may crack it even if you hash it millions of times.
Besides, hashing it multiple times won't make it more secure because at the end of the session you will need to disclose the unhashed server seed and how many times it was hashed so the user can verify it's the one you really used.