YANBU. I agree that it's inappropriate and, even if well intentioned, shows a lack of consideration for you and proper boundaries. You're right to get upset, the key is just to not let it become a huge upsetting drama as it doesn't have to be.
I think you should say something to your ex partner. We teach people how to treat us. They may not always like it but does set things right. It's okay and you're allowed to set healthy boundaries with people. If anything healthy boundaries create better relationships so it's a positive thing to set them.
It doesn't have to be a big confrontation or a row. I think maybe just mention when you next see him or in a message if more comfortable something like this:
I'm certain that it was written with love and only the best of intentions and it's really not a big deal, but I noticed your Valentine's card to DD inadvertently suggested your partner is DD's mum. I'm quite sensitive about this and I feel it's confusing for our DD being adopted and already having more than one "mum" in some ways. I would really appreciate your sensitivity about this and finding a good solution. I was thinking there are other lovely ways to sign off cards etc that would work great eg love Dad and (Jane). Thanks so much and lots of love.
Obviously in your own words. But this can be done in a non confrontational way, the key is to prevent getting sidetracked into pointless and unwinnable arguments about their intentions and motives. Just focus on the facts and what needs to be different.
If they genuinely meant no harm, they will likely respond constructively. If they react angrily and defensively, then you will have flushed them and their bad intentions out into the open and can know what you're dealing with and to be on guard in future.
Good luck and hope this helps.