Head of Let's Encrypt here.

We considered introducing a site seal because it's a common request but we've decided not to do it (at least for now) for reasons similar to many in this post.

It's hard to design a seal that accurately conveys the value added to a site's security by a CA, and the potential for abuse is high. A CA seal either means nothing or implies too much because having a cert from a trusted provider is just one part of what it means to be a secure website.