Sketchfab model embeds are supported in Tumblr posts.
To add a Sketchfab model to your Tumblr, first, create a new Text post. Click the Settings gear in the top-right corner (as seen below) and choose HTML as the Text editor.
Copy/paste your model's embed code from the Sketchfab model page into the body of your update. And then click Post!
Keep in mind that switching back to Markdown or Rich Text may break the iframe code, so you should stay in HTML mode or finish the rest of your post in another mode first.