It is important in our relationship but overall, no, I don’t think it’s vital to an FLR.
I was cuckolded by my first wife for a brief period of time, but she was never into dominating me. We ended up splitting up with a lot of the reason being I needed a woman who would dominate and control me.
My current wife and I talked a lot about this before we got married, and our marriage has always been an FLR. She has a very dominant personality and that was one of the things that attracted me to her. She was also very promiscuous, she liked variety in her sex life. She knew my first wife had cuckolded me, but she attempted to remain monogamous after we got married. (She had sex with another guy with my knowledge for two months before we got married).
About two years into our marriage we tried swinging for a while, but her jealousy was a problem. I also never really enjoyed having sex with t other women, but loved seeing her experience a new sexual partner. We talked about it and agreed that her cuckolding me was what we both wanted. Her having the freedom to do as she pleases while I remain faithful is an integral part of our relationship and her dominance over me. It works very well for us, but it certainly isn’t a necessary part of an FLR.