Didn’t we attack Hillary Clinton or Big Mike Obama? 
Hillary Clinton didn't just insert herself into the campaign to promote her husband - she actually said it was a "2 for 1" deal with her husband because she would help him govern. At that point, her political views became an issue. That wasn't the case with Michelle Obama, Melania Trump, or Casey DeSantis.
I don't think the purely personal attacks against politicians, much less their wives, are appropriate period. I thought calling Hillary "Cankles", and the stuff said about Michelle Obama being a tranny, or man, or whatever, was low class and uncalled for. Still don't like it. I think a lot of people who aren't highly partisan have a negative view of those who make those kind of comments, and I don't think it is helpful politically.
Obviously, it's worse when the people making those comments are media types or have a real public platform. Jokes made in private or small online communities are less of an issue because they don't get disseminated into the public.
That being said, the people who are in the wrong are the people who said those things. It isn't any more right to make those kind of comments about Casey DeSantis then it was to make those kind of comments about Hillary Clinton or Michelle Obama, nor does the fact that some people said those things justify comments being made about Casey DeSantis. It's not her fault that other people lacked class.