Trending topics is a thing on mastodon.social so that's cool! :)
They are in the Getting Started menu, but when your screen is too short (like mine) they disappear.
If your screen is too short then the only way you can get to them is by navigating to http://mastodon.social/web/trends to open the "Trending now" column - and it's not pinnable yet, nor is there a link to it anywhere in the UI yet.
Hopefully it'll become more accessible soon. :) So far it's great though!