Yes you can - It is your number of MHOs divided by the opinions you have given on questions where MHO is awarded after you give your opinion multiplied by 100 that gives MHO %. Don't worry though you would have to hit a real long spell of answering questions and not getting MHOs to drop.
If you mean MHO% of course you can. Your MHO% is how many MHOs you receive out of all opinions. Once you have any MHO% as you continue answering, if you have opinions without getting any MHOs, it will lower your percentage.
If you answer 10 questions and get 5 best answers you will have 50% best answers. However if you answer another 10 questions and get 0 best answers, your best answer ratio has changed from 50% to 25% so yes you can lose MHO.