TL;DR: Mail delivery using TLS and signing using DKIM are weak protections, compared to accessing a web site using HTTPS. They should not be assumed to provide the same security and the indicators better should not suggest such interpretation.
... delivered over TLS?
Mail is delivered hop-by-hop between client and final mail server, i.e. there are multiple servers in between. TLS cares only about protection between these hops, not on these hops. Each of these servers has access to the plain unencrypted mail. The indicator in GMail shows only if the last hop of delivery was done over TLS - and this is all what the final mail server can control.
Because of this hop-by-hop even a TLS indicator does not mean end-to-end protection of the email, i.e. from sender to recipient. Contrary to this HTTPS is end-to-end protection from browser to server. See TLS encryption email for more on this.
... signed by DKIM
DKIM does not provide a cryptographic signature created by the sender - this would be done instead using PGP or S/MIME. DKIM is done instead by a mail server on the way. All it shows that the mail was (mostly) unmodified on the way from this mail server to the final mail server. It does not indicate that the mail was sent by a specific user, nor that it was unmodified between sender and DKIM signing mail server.
Note also that DKIM actually allows modifications of the mail. Depending on the method changes in white space are allowed but there can also be changes which completely change the interpretation of the mail - see Breaking DKIM - on Purpose and by Chance for more on this.