I have been playing on an expert mode, small world and I just got to hardmode, and I would really like the star cloak and the Titan gloves. I have set up a farm at 350 blocks deep (in the caverns) but I cannot find any mimics! Please help!
The location for your farm is fine, but based on your picture, it doesn't look like your farm is big enough. A real mimic farm requires several screen lengths of completely cleared area, otherwise there's a strong chance the mimic will just spawn in a location that isn't a part of your farm (such as where that jellyfish is in your picture), which means you'll never actually farm it.
Additionally, if you want to know if a mimic is close by without manually searching for one, make sure to check with the Travelling Salesman whenever he's in town. He sells the Lifeform Analyzer, which displays the name of any rare enemies on your screen.