It’s not the word “lighter” that’s the issue, it’s the word “less”. If I say something weighs 80% less, … you know how much that is. 100% less, it weighs even less – nothing at all. 500% less (i.e. 5 times less), suddenly it weighs more?

“one fifth the mass” is not the same thing as “five times lighter”

Consider something that weighs half as much. It’s 50% lighter … 0.5 times lighter. Something that weighs 0.2 times as much has 20% of the weight, and is 80% lighter. If it weighed 1% as much, it would be 99% lighter (0.99 times lighter). If it was 100% lighter … it would weigh nothing. Five times lighter would be -4 times the original mass.

We already have accurate and precise ways to describe less mass (albeit leaving aside for the moment the distinction between mass and weight). It’s no harder to say “one fifth” than “five times”, but only one is correctly describing what is going on.