I must sign a "Trademark Licence Agreement for Ubuntu" in order to use their logo in the sticker. But there are some points in it I don't understand.
"License grant. Canonical grants to you a non-exclusive licence to use the Trademark as described in Exhibit A to: (i) promote Ubuntu, (ii) participate in the Ubuntu community, or (iii) contribute to the Ubuntu community....."
but with the sticker I'm "promoting" another community/project independent from ubuntu
"Restrictions. You may not do any of the following without Canonical's prior written consent: [...] use the Trademark in conjunction with any other name, word, logo, or symbol, (iii) ...."
so seems I can't use ubuntu logo in conjunction with the Yumi image ( it can be defined another logo, or symbol) until I ask (another) written consent
"Trademark Statement. You agree to include the following disclaimer, or similar wording, where appropriate: “Ubuntu is a trademark of Canonical Limited and is used with the permission of Canonical Limited. We are not endorsed by or affiliated with Canonical Limited or the Ubuntu project."
that is impossible to do in the sticker pack
so I think it is a no-go for my use of ubuntu logo on the yumi sticker.