I prefer holster slots/loops the same size as the belt I'm using, but sometimes that depends on the gun and/or holster.
For example, I have a Side Guard Holsters Quick Clip IWB holster for my 642. It uses a metal clip sized for a 1.75" belt, but I've used it with 1.25" and 1.5" belts. A couple of reasons why this works is that the 642 is a lightweight gun and the holster uses a rough-out construction, so fore-aft shifting is minimized. It still happens, but doesn't impede accessibility, comfort, or concealability, and typically settles into place.
My EDC is a Beretta PX4 Compact in a Kramer Handgun Leather IWB#3. I use the 1.25" loops with a 1.25" belt. The loops are not a tight fit, though. Fore-aft shifting has not been an issue as it is pretty minimal. It does become an issue if I use the 1.5" loop setting with a 1.25" belt. I haven't tried the 1.75" loop setting with a 1.25" belt, but I imagine fore-aft shifting would be more pronounced.
So, to answer your question, a belt/loop-slot gap would bother me if it allowed significant shifting to occur. As long as the arrangement is stable, I'm good with it. But it's going to be more stable when they match up.