As a private citizen, she absolutely has an unquestioned right to stay at home and not take part in this election.

However, she is in fact working hard this election cycle as per this article,

Michelle Obama has worked behind the scenes to boost turnout in key swing states

So that should be enough. No one has any right to ask more of her.

Also,

she has expressed fresh concerns about security following two assassination attempts against former President

So I totally get why she wants to avoid doing this one specific thing. That’s fine, she’s already gone over and above in my opinion.