I think that’s a fear that a lot of new Tumblr writers have. I was definitely scared of the same thing at first, but you have to remember that there are so many Tumblr users who have different tastes in writing. And with so many different preferences, there’s guaranteed to be several people who enjoy your style of writing.
I’ve had this blog for about two years now. For a starter, I would recommend connecting with other Tumblr writers by following them; you can ask them for writing advice too. Some of them provide feedback on other writers’ work, so you could ask them if they would do so for you too. (But not all of them do that.)
I have a list of great Tumblr writers that you can follow here.
That will probably help you start off on the right foot.
Good luck! :)