I am assuming the person means posts like” This is what I made for XX” and then a pretty picture of it. It’s not about being selfish. A tumblr is like a scrapbook, where you post pictures of things you like to remember; things you made, thing that made you lol or cry. A simblr is usually a scrapbook of the things you made in Sims: pictures of your game, sims or CC you’ve made. It’s for the owner of the simblr to decide what goes into his/her scrapbook, not the followers.
Tbh, I think the same goes for any blog or LJ or whatever.