Helmets are tested in a variety of impact scenarios without modification. Who knows how some random attachment might negatively affect those outcomes. Presumably you might use some sort of adhesive to attach things — that adhesive can damage and weaken the shell of the helmet.

I just use my brain and assume that the plastic 'candy' shell on the helmet doesn't do anything (other than moisture protection) and as long as the styrofoam innards are in-tact, then nothing materially changes.

Otherwise we'd also see warnings against stickers and artwork on the shell of helmets, but we don't. Or at least I didn't, but I don't really read manuals.

Every motorcycle helmet manual I’ve read explicitly says not to put stickers on the helmet. Protecting the foam from moisture is important — a rainy ride can significantly weaken the foam. Eg: https://www.bellhelmets.com/on/demandware.static/-/Sites-bel...