This Voltaire quote is misleading. He continues: “… one would imagine this piece to be the work of a drunken savage. But amidst all these vulgar irregularities, which to this day make the English drama so absurd and so barbarous, there are to be found in Hamlet, by a bizarrerie still greater, some sublime passages, worthy of the greatest genius. It seems as though nature had mingled in the brain of Shakespeare the greatest conceivable strength and grandeur with whatsoever witless vulgarity can devise that is lowest and most detestable.“

So while he is pointing out the crassness of the work, he does think it’s great. Voltaire was very interested in the English, and pointing out the contrast here was definitely coming more from curiosity or even admiration than from dislike.