It is bad manners to make fun of someone else's voice. Even worse, it can jinx your own performance. So don't do it - ever!
I don't care if your best friend's creepy brother staggers up to the karaoke stage and belts out an off-key version of "I Want to Be Happy." Do not make fun of him! Don't heckle the poor guy during the song. Don't whisper evil things about the performance to your friends. Don't even gleefully run the on stage catastrophe like a music video in your mind's eye and chuckle in private.
Why? Because if you do, this behavior will come back to haunt you when you are feeling vulnerable. And singing can definitely make you feel vulnerable.
When you taunt or laugh at someone else, you're training your left-brain critic to attack not only that person, but also yourself. Even if you are proud and strong while laughing at someone else's expense, the tables can quickly turn. Someday, perhaps sooner than you think, you will have an important audition. And that morning you might, for example, discover that your kid sister found a box of old love letters you kept from junior high and read them aloud in the school cafeteria.
After yelling at her as loud as you can, you will head off to the audition feeling strangely exposed, angry, and fragile. Then, when on stage and only four bars into your song, you will see the music director lean over and whisper something to the choreographer. You will imagine they are talking about you the way you've joked about other singers; Every horrible thing you have ever said about the human voice will come thundering back out of the darkness to envelop you, and your well-practiced audition number will unravel right there in front of everyone.
So be charitable when you speak and think of other people's voices. The act of singing moves you into the realm of "your dream life" - where that monster under your bed still lurks. Even though you are older now, it can still be a very dangerous place. Especially if you've allowed your inner critic to practice hurling insults at everyone else on karaoke night.