Yes, there is a Webmail portal for Thundermail, though it is still a very early alpha product at the moment and our support for it is currently limited.
You can access your account's Webmail at any time at webmail.thundermail.com.
Development has been very active, with changes happening all the time! If you would like to see what's being worked on, you can check out our Webmail repository on GitHub.
If you have thoughts on how to improve it, please visit Thundermail Ideas, our feature request board. Search for "webmail" in the "search for posts" bar to see if your idea is already there to upvote and comment on. If it doesn't exist yet, please click the "Share Idea" button to let us know what you're thinking.