A lot of generals made dumb moves in the civil war even those educated at West Point and with hindsight it tends to amplify them ten fold , Sickles movement on the second day is no worse than Lee , Grant , Burnside , Hooker , Hood etc etc.
I think the fact that Sickles bragged that his movement won the Battle of Gettysburg and stopped Meade retreating really rubs people up the wrong way and I get that , And yes from a military viewpoint the ground he chose was absolutely not textbook defensible leaving his flank in the air and a gap in the line however had Sickles gone back into line we just don't know how it would have played out Longstreet might have executed his flanking move and rolled up the Union line we can speculate but we just don't know.
Gettysburg is one of those battles that has so many what ifs more than any other battle I know and that's what's so fascinating about it and why its the most studied battle in the Civil War even though the outcome really didn't effect the war that much.