If you are out in the congregation and know the music, why not sing?
I’m sure the Lord is smiling and pleased with the person who sings off key compared to a choir member who is well trained but not in a state of grace.
I have sang in many choirs since I was a child. I have seen things.
So you should start singing even if no one else in the congregation is singing? ... and create even more of a distraction than if you were just singing off-key (with butchered Latin) very loudly, in someone's ear?
So, this attitude of somehow the Lord being pleased by you exercising your vocal chords (even if you're ruining the dignity of the Mass, etc.) ... is actually contrary to what St. Pius X stated, that you should rather have a Low Mass than to do a High Mass BADLY. If I were in a congregation and no one else in the congregation was singing, I would most certainly NOT just sing because I "know the music". But perhaps you left that qualification unsaid. So I'll assume you meant in a situation where most of the congregation were singing already. But, even then, no, it's not OK to just belt out cacophonous nonsense (buthered Latin).
I think the entire presumption and premise of what St. Pius X said was that it should be done well. That's why he mentioned having instructors go around helping the laity do it correctly. Of course, it's not just necessarily a question of training, as some individuals are simply tone-def, and teaching the correct pronunciation of Latin might be a challenge. For St. Pius X, Italian isn't THAT radically different from Latin (at least in terms of pronunciation) that it would present a problem, and, as I said earlier, Italians just seem to genetically be able to sing, even before they learn how to walk.