It may be that the Democratic party is trying to stay away from ad hominem attacks/fallacies.
This is the kind of thing that can be simplistically described as an attack on a person because you don't have anything real to counter their arguments.
Person A: Climate change is drastically changing how the weather interferes with business and even residential care.
Person B: And how would you know that. You aren't a scientist and you're just a dumb blonde, anyway.
In this instance, Person B is using a personal attack instead of refuting the claim. This would be an ad hominem fallacy, since they aren't saying anything they can reasonably prove or has real relevance. Even if the person isn't a scientist, that doesn't mean they are clueless to what real scientist are saying about climate change.
An example response that wouldn't be a fallacy, but would still be an ad hominem attack could be:
Person B: You have a long history of misinformation and you constantly publicly support conspiracy theories, so why should we believe anything you say?
This would have to be backed up by facts that the person does actually believe debunked conspiracy theories and other misinformation, otherwise it's still a fallacy.
In the case of the presidential candidates throwing around claims about the other's intelligence, mental capacity, or other personal attribute, that can be considered an opinion, which can be refuted. One party can provide "evidence" of the other's mental incompetence and the other party can provide "evidence" to the exact opposite. Because it's so subjective, it's considered the 2nd lowest form of attack.
The lowest form of attack is name calling, which Trump also does, but I digress.
The Democrats appear to be sticking to things that are pretty cut and dried when it comes to provable results. When they talk about China relations, they talk about the sales and other financial losses America has seen because of how Trump handled those foreign affairs.
Even when they do attack Trump for being "dangerous, negligent, authoritarian, unlawful, racist, lying, and incompetent", they are still pointing to specific instances where it's fairly obvious.
They talk about how Trump attacked AOC and other Congresswomen for their heritage and calling Mexicans "drug dealers", "criminals", and worse when they say he's racist.
They talk about the whole "grab'em by the pussy" incident when they say he's a misogynist.
Democrats also point to specific instances when he outright lied about a number of things or specifically about the coronavirus.
These are still ad hominem attacks, but because they are factual and not based on bad logic, they aren't fallacies. Because it's considered "dirty politics", as another Answer mentioned, it should still be avoided. They feel they can get away with it because the ad hominem part isn't the center point of the advertisement, but rather the evidence is. On the other side of the aisle, the Trump campaign focuses on the ad hominem attacks as the entire ad.
As an example of this opinionated form of attack ads put on by Trump's campaign, I just listened to an advertisement on my current favorite internet audio streaming site. It was a collection of Trump supporters talking about their fears of Biden and how good they think Trump is doing. The fact that what they say can be easily disproven makes this an ad hominem fallacy and very bad form of advertising.