A security seal is not appropriate: Certification seals are designated for classes of certifications in which the qualified Certificate Authority performs an audit of the business, charity or government entity and sometimes the person(s) responsible for the account. Letsencrypt certifies that the site is the domain that is listed as by ICANN as the domain owner. It does not do any verification of who is managing the site or what the site is used for. Letsencrypt is able to offer the service for free because they have not issued any assurances to a website’s visitors or customers that the site is owned by a specific company or is reliable or any other commercial assurance.
Therefore, a site seal beyond the green lock in the browser is not appropriate and is unlikely to be provided.
That does not prevent a site owner from unofficially making use of the Letsencrypt logo to insinuate that the site security is similar to a commercial cert that does the required business and/or personal identity audits.
“We want it all for free!” Letsencrypt, as far as I understand it, will not offer personal or business class certifications because that costs money to perform and maintain that goes well beyond an automated server. What I hope happens is that LE increases the competition among certificate authorities to make getting web commerce and other certs easier, more easily renewed, and much cheaper.