Its really fine if you dont want to. People dont care. Enjoy staring at your lovely treecko or anything you didnt evolve. Its good that you managed to raise it to lvl100, since it would be easier with your higher stats. Evolved pokemon are usually stronger. Like if you see a onix evolve into steelix the defence skyrockets.
But in competitive play, those cute pokemon will mostly not stand a chance against the hundred of strong evolved pokemon. So if you are not going to link-up with friends or something, keep it unevolved as you wish. If you call pikachu cute, he could be good with a light ball which doubles its special attack.