"Scorner" is the strongest negative term which the OT uses to describe the wicked. It is diametrically opposed to the faithful (Psalm 1:1,2). Not only does the "scorner" choose the wrong way, but he mocks the right way. He is not merely misled, but he delights to mislead others. This is very opposite to the wise man, who understands the order of things (Prov. 15:12; 21:24; 22:10; 29:8). Calling a believer a "scorner," just because you don't like how he says things, is extremely serious, much worse than if you called them an "idiot" or a "knucklehead" or "ignorant."
It is serious because it perverts Scripture. Doing some of the same things that a scorner does, does not make a person a scorner. For instance, Proverbs 21:24 says: "Proud and haughty scorner is his name, who dealeth in proud wrath." Someone who exhibits pride and gets angry is not necessarily a scorner. Not to defend pride or selfish anger, but sometimes they are actually confidence and righteous indignation. Sinners don't like strong convictions about and anger over their sins. Proverbs 3:34 reads: "Surely he scorneth the scorners: but he giveth grace unto the lowly." Someone may perceive another not to be lowly enough. When making a theological point, he does not fold under it in submission. That might be because he confidently disagrees with it. He may offer Scriptural proof against it. That would make everyone that disagrees with us a scorner, and that isn't true. God knows that the scorner is someone who rebels against Him and His Word, not human opinion. When we look at all the passages on scorners, we find that the scorner is someone who rebels against God and won't listen to His preachers. When we alter the meaning of the word "scorner," we twist God's Word.
It is also serious in that it slanders people and even worse when it is God's people. A real love for Proverbs is seen in people who use it to obey and honor God. It helps them walk in wisdom, pleasing the Lord. It isn't for selectively labeling people. "You're a fool because Proverbs says a fool....." You're simple because Proverbs says that the simple...." "You're a scorner because Proverbs says that a scorner...." Satan is an accuser of the brethren. Satan also uses Scripture (cf. Genesis 3; Matthew 4). Adding Proverbs to a false accusation doesn't lend authority to accusation; it blasphemes God's Word.