Do bochurim dress in shorts and Crocs on Sundays? If they don’t, then it is probably because, one, they are conscious of representing their Yeshiva, two, there is peer pressure against it, or, three, they see themselves as standing before Hashem at all times. If they dress down while camping or digging in the garden, I can understand.
Tznius is a matter of dressing with dignity and a degree of self-effacement. In the secular world you can see both the extremes of sweat pants (I dress only for my comfort, and I don’t care if I look like a slob) and to grab attention such as florescent colors.
Crocs are OK on Tisha B’Av and Yom Kippur, but I wouldn’t wear them outside the house other days.