Mustard is used as an easy “natural” emulsifier to keep things together, since the ingredients in mayo don’t really want to be in that configuration and it is easy to “break” that mix.

The amounts needed to do so is very small though, and usually imperceptible at the scale of a whole jar of mayo. Definitely not something you’d notice in most cases.

But again, every brand is different. Some probably are more mustard forward for whatever reason.