@stefanie uhm... not sure! mstdn.ca sells Mastodon Merch using a variation of the logo! I think the logo is trademarked, not CC0 or CC-BY-SA or something!

According to the Mastodon gGmbH you'd need to make a request here: https://joinmastodon.org/trademark in order to "use the logo not specified by trademark policies" which includes merchandise. I believe mstdn.ca was able to do this mainly because their logo looks entirely different to Mastodon's (red color, different letters/font, etc.)

So: No, they are not free to use. The shirt I am wearing is official Mastodon merch!