Perfectly okay with a crescent of crisps on the side of the plate, perhaps with some salad or cress, particularly if it’s a sarnie from a fancy cafe and they’re serving up some artisanal crisps that you normally only find in service stations.
It’s just not right putting them in the actual sandwich though. The clash of textures is all wrong, and crisps don’t really go with butter, so it’s even worse if the bread is buttered. Just eat them separately eh.
Chip butties are another faux pas. It’s just the height of blandness. There are very few instances where it’s fine to combine carbs with carbs, and neither a crisp butty nor a chip butty qualifies IMHO.