By using profanities in a debate the person will cheapen their point and make it seem more forceful and aggressive, leaving any impartiality at the door.
The use of insults in a debate means that the person has given up on their point and instead if trying to defame the character of the opponent, it is a tactic we seem employed by a lot of social justice groups and individuals.
Using profanity in general implies that the person using it must resort to using emotionally charged words in an attempt to win others to their side.
One mark of a good debater is the ability to shrug off any emotional or personal attachment to a particular argument. In doing so, that person can then assess the argument from both points of view and choose the point of view that makes the most sense globally based on the parameters of the debate, rather than making the most sense for that person alone.
The only time that using profanity in a debate does not necessarily mean that the person is a poor debater is when that person uses the profanity in order to prove a point about the debate topic.
For example, in a debate that has a main point centered around the use of profanity (this one for example), the person arguing a position could use profanity in their argument for the sole purpose of proving that profanity in general is good or bad.
Profanities and banter can be useful methods of expression, depending on the context and intent (and skill/playfulness used). They can be fun ways to exchange ideas too. Very often, however, they are used as a method to deflect facts that are difficult to refute, or used as a desperate/weak attempt to counter a strong argument. Overall, most people will resort to swearing and insults when they cannot win a debate or argument without them. Furthermore, some people don't know how to express their point of view without aggression due to problems in their childhood history. An over-reliance on profanities and vernacular insults is a serious limitation to an individual and their ability to express their thoughts and feelings in my opinion. Furthermore, I doubt such people are very happy and/or content in life.
This question is subjective. You might feel someone has poor skill anywhere in any conversation just because they say a word or phrase that is seen as "negative" in society. For example, "consarn" is a word that replaces "Goddamn", but it sounds like some Indian cuisine. However, swear words might be emotionally appealing to the audience so it intensifies their side of the argument.