I currently have a custom subdomain for my tumblr and this surpassed my actual website in seo ranking. Is their a way to reverse this?
All you have to do as of now is focus more on your actual domain, this happens in scenarios where you are actually working and updating your sub domain on regular basis.
You shouldn't have optimised Tumblr for the keywords you're after. However, it can also happen accidentally. A Tumblr presence (rather than spam) is good to have, but it's high authority and can get a lot of completely whitehat backlinks quickly. You want to make sure any links you are getting for Tumblr (reblogs or whatever) are linking back to your main site. largely, Tumblr will take care of itself if you post something there. Just focus on on your main domain; strong on-page content (for humans, not SEs - a lot of the old on-page SEO tricks no longer work, thankfully), but also your backlinks need good on page. The next time you go to make a like, you need to carefully and objectively assess: is this a good page (well written content and relevant to your site), is how the link will be placed relevant (e.g. within some kind of content on the page)? Good luck!