Judas was a valid bishop. He has nothing to do with this question. There is no doubt about his validity.
It is not a sin to say a priest is doubtfully ordained and therefore we should not go to his Mass. Telling the truth is not sinful.
You do not have the authority to say this.
You may say: "Far as I am concerned" or "In my opinion" he is not a priest," or that "his validity is doubtful" and "I will not go to him."
But you cannot go around proclaiming invalidity or doubtful validity as though it has been officially decreed and as though you have the authority to say it - because you don't, or as if it is a dogmatic fact - because it isn't, and if you're wrong, and you could be wrong, you will suffer the consequences of being wrong.
For example if I say that Fr. Pfeiffer is very doubtfully a bishop and therefore men ordained by him are most likely not priests but laymen dressing as priests that is not a sin. And the young fellows he ordains are guilty of asking orders from him. Anyone receiving the sacraments from them is foolish. It is silly to go to confession to a man who is not certainly a priest. Because if he is not a priest I remain in my sins. And it if it not certain that he is a priest I don't know my state. Am I still in my sins or were they absolved? I don't know. That is why we must tell men who we don't know if they are priests or not to get conditionally ordained by a certainly valid bishop.
While I personally agree, you (we) could be wrong. Aside from that, everyone here already knows the repercussions of doubtful or invalid priests and bishops, so no need to keep repeating it.