BS BS BS.... you just want her Mom to think 'oh isn't Amy just lovely? Oh I wish Amy was still around'.... it's a passive aggressive game play and your ex AND her Mother will see right through it. Get some dignity and move on. 

No problem.

But on the card just say ‘Thank you for your kindness ‘ 

You should do it! If your ex reacts angrily, just calmly respond to her with compassion, sweetness and understanding, while also standing your ground. “I understand you’re upset, after you left me for another man. It bothers you that I haven’t just gone away quietly. However, your mom and I always had a good time talking, so I just wanted to let her know that I appreciate her and what she does. She’s a good person and deserves some credit, that’s all. I wasn’t trying to talk to you whatsoever”

Nothing wrong with sending her mom flowers.  But make sure her mom knows that it is only for the fact that you love her, and appricated her in your life, however briefly and brekaing off with her daughter does not mean that you have stopped caring about them.  Don't even mention her daughter's name.

Sending your ex's mom flowers as a signal to your ex is wrong. Sending your ex's mom flowers for a mother's day or any holiday gesture between you and your ex's mom is acceptable with a short personalized note that you miss your ex's mom and the breakup is between you and her daughter. 

Don't use a third party as attempt to get back with an ex.

If this is not just a sign of respect to your ex's mom, don't do it.