Hi Felix,
Since STEEM is a decentralised system, responsibility for the various SEO tasks are spread across the different people involved. Each DApp creator can perform on-page SEO in order to improve the ranking opportunities for STEEM users and for the site itself. Each STEEM user can also optimise their posts for their target keywords and use many of the common SEO tactics for on-page optimisation.
Managing Off-page SEO is something that can be shared by the DApp creators and the STEEM users too. The more aware the STEEM community is of good SEO practice, the better the results will be. The process will need to be overseen by the DApp creators using Webmaster tools in Google and Bing for best results.
So in summary, there needs to be a co-ordinated team effort for STEEM to achieve the best SEO results. So far we do not see this taking place on STEEM at all. In future we intend to post on this topic in more detail and perhaps co-ordinate some community activity to improve STEEM's search engine performance.