Yes, the old name for Sour Patch Kids, "Mars Men", is sexist. The name of this candy could have been "Mars People," "Mars Beings," or even "Mars Kids." However, due to the stifling patriarchy in which we all suffer under, the name of these delectable candies was deemed to be an homage to men.
First, it's not sexist. "Mars men" has been used to describe potential martians for years. There's no reason to assume that they simply refer to the male gender. It's like things such as "crewmen" and "mankind", among others. Would it be sexist if it was "Mars Women"? No. There's no reason to cry wolf on this, when there are larger issues to be talked about.
The name Mars Men is not a sexist name, but could be called outdated. To appeal to a wider audience, the company opted to change the name to Sour Patch Kids. Companies are often terrified of being accused of discriminating. Even if the company did nothing wrong, they still suffer financially. So most err on the side of caution.
No, I don't think that calling a candy Mars Men is sexist. It was a catchy name that was probably used because of its alliteration. In general, people today are way too worried about being politically correct about everything. It's like everyone walks around just daring someone to offend them.
People shouldn't read too much into things, like a name that candy had back in the day. The truth is that there are two different genders, and people who are transgendered or questioning their gender. There is nothing wrong with acknowledging these things when naming a type of candy. It's not that significant.