Have you ever tried to make a mug mockup in Adobe Photoshop and spent time getting your vinyl decal or dye sub design onto the mug, put it in an amazing background where it looks like it's sitting on a table, and then the shadow ruined the whole thing? It looked super fake and maybe the mug even seemed to be floating in the image? I hate that! LOL :-)
Making a realistic shadow in a product mockup requires more than just using the drop shadow effect in the Photoshop layers palette. It actually requires a little observation about how shadows look in real life, and a few layers with some deftly applied blurring to really look convincing.