It depends on the fish being fed. If they are tropical fish and swallow the frozen food block/ cube in one bite, it can drop their body temperature a little bit and might cause problems (it doesn't normally but might). If they are coldwater fish in a coldwater tank, and they swallow an entire cube of frozen food it's not normally an issue.
If you have small fish, letting the cube float on the surface can allow the fish to pick at it and pull little bits off to eat. However, if it doesn't all get eaten on the surface, it can sink to the bottom where it might not get eaten and will cause water quality problems. If you know the fish won't eat an entire cube while it's frozen and floating, defrost it first and offer a few bits at a time until everyone in the tank has eaten their fill.
You can use a pr of scissors to cut frozen foods into smaller pieces for smaller fish.