U-joint cups should be an interference (press) fit in the yokes so the cups cannot move or rotate at all within the bore. If the cups are loose the yoke needs repair. I recently repaired a driveshaft yoke with a loose U-joint employing a neat trick to reduce the yoke ID.
Here's the procedure. Use a battery charger to deposit (weld) tiny spots of metal on the inside of the yoke bores. I used an older style 12 volt charger containing a traditional heavy transformer. Clamp a thin steel wire, nail, oxy welding rod, etc. in some locking pliers and attach a lead to the pliers. Ground the yoke and start touching the rod to the inside of the yoke bore. Spark, spark, spark, many many times until hundreds or even thousands of little tiny pits of metal are deposited. Careful, the wire gets hot. A couple times per second will add up quickly. The result was a very tight press fit cup into the formerly loose bore.
Additionally, when a U-joint is properly installed the cups should fit snugly up against both snap rings so the joint is centered in the yoke. If the joint cups are slightly too wide, thinner snap rings may need to be used. If the joint is too narrow, washer type shims can be placed against the cups.