Just a healthy opinion here, certainly not a criticism of their relationship, but I don’t think partners have to love or participate in all of the other person’s passions. I think it is healthy to have differences. Otherwise, to force the other person to “pretend” to enjoy it is worse. Then you are putting the pressure on them to do something they really don’t want to do which creates resentment.
I like to knit. I don’t expect my partner to take it up or go with me to every yarn shop or fiber show; which, to date, he only took me to 1 and it was a local show. And I didn’t make him so much as I brought it up and his immediate response was a lightly chuckled, “No.” A couple of beats passed before it dawned on him I had gone to dozens of events of his choosing — things he knew I attended only because he wanted or needed me to go. And yeah, my feeling were hurt a little considering all I had done for him in terms of going and helping out at his events. He immediately changed his mind, saying he would take me. I knew he didn’t want to go; I knew he was just doing it for me. I gave him an out — several times, telling him he didn’t have to and I knew he didn’t really want to. Also, I was mindful I wouldn’t be able to linger there long before he’d be bored. Had I gone myself or with my friends who are into it, I could have stayed a couple of hours. We went together, and I didn’t enjoy it. I wish he had just taken the out instead of feeling obligated to go.
I know how it feels because I’ve attended dozens of functions associated with his passion project, doing the supporting partner thing. I do it because I love him not because I’m in love with the thing he’s currently into. This involves some out of state travel and hauling a lot of heavy gear, set up, and chatting with people (at times for 2 or more days) at expos. I’m an introvert and don’t especially like chatting up strangers, but I do it for him and his passion project. But by the 3rd day, he knows my threshold for losing sleep, hauling heavy stuff, and dealing with people for 10–12 hours, has withered to exhaustion.
Point is, I don’t try to make my partner do the things I love (not saying your dad is) or certainly not as often as I like to do them. And while I don’t have to help my partner with his passion project, I do it for him. No amount of him wanting me to love it is going to make me love it. There is nothing wrong with a partner not being into something. That’s what friends are for.
Your mom should get some credit for the times she does spend time with your dad and his car passion. But it shouldn’t be to the point where he’s heartbroken that she won’t join him — it’s not the thing she is passionate about and it’s not fair to expect her to be. Oh, but I do agree, I don’t think it’s healthy to (playfully or otherwise) chide, mock, or scoff about the other persons' passion.