It’s okay to be okay with it, it’s even better when there is convincing evidence. I’m just saying you can skip the fallacy by attacking their argument/lie, which you have to do regardless if you want to conject that they are liar.
However it still implies that they are a liar in some habitual or further-reaching sense. This is not easy to prove. Did they lie before? What were those lies and how can you prove them so? Will they lie in the future? How can you know for sure? These are the questions that make it a fallacious label as it frames character rather than argument, and it just seems a bit … dull and irrelevant, when you can attack the lie just as easily.