Hey
why is there no function in tumblr follow module like: Follow all notes of a given tumblr blog?
Because there were no requests for it 
looool in my experience this is the only method to gain traffic by following people that have liked/reblogged posts of certain tumblrs. It should work like that: Follow all notes of tumblr account. You put in the blog url - mp scrapes all notes of all posts and follows. Do you think you can implement this?
If a lot of users have requested for it, the Devs will add it to their list
do not think many use the tumblr module
To be honest, not as much as the other social networks