"Slipping up" means being sloppy, or not thorough about work being done. Ice is a slippery substance. If someone is not careful when they are walking on ice, they can slip. To slip is to lose one's balance or footing. This joke is playing on the word "slippery" and how it relates to poor quality, but also falling on ice.
The ice at the rink has many ruts . I think the maintenance crew is slipping up .