This issue fixed in JComments 2.3.0... If you're still using 2.2.0.2 you can fix this:
1. Open file /components/com_jcomments/js/jcomments-v2.1.js:
2. Find code
updateCounter: function(){if(this.counter!=null){var ta=this.ta,e=this.counter.e;try{var n=document.createElement(e.tagName),v=this.counter.max-ta.value.length;n.innerHTML=(v>=0)?v:0;e.parentNode.replaceChild(n,e);this.counter.e=n;}catch(ex){}}},
3. Replace with
updateCounter: function(){if(this.counter!=null){var ta=this.ta,e=this.counter.e;try{var n=document.createElement(e.tagName),v=this.counter.max;if(ta.value.length>0){v=v-ta.value.length;var ln=ta.value.match(/[^\r]\n/g);if(ln){v=v-ln.length;}}n.innerHTML=(v>=0)?v:0;e.parentNode.replaceChild(n,e);this.counter.e=n;}catch(ex){}}},