No it's not. It isn't for the dad or his own feelings towards her, he recognizes his kids want to be able to do something nice for their mom for mother's day, and helps out. It's so they can happily go up to mom with nice things for her and feel good about it. Not so he can try to be better than her, or try to flirt with her.
My dad also did this. He always went the extra mile with me and is the most genuine kind person I know. He'd also help me get small gifts for my half-sisters for when I got came back from visiting. It's not about romance or being a doormat or anything, it's about helping your kids show affection to their mom not your ex.
And if they see him doing this, while feeling happy about it, and then see their mom not doing the same back, they'll grow up thinking he's a better person than her, because he is.