Well in Viktor's case it's more coincidental design than an intentional mark.
Vi's comes from a time before she can remember.
Jinx's is basically the X in her name and acts like her signature.
Ekko's weapon is the seconds have from a clocktower, so it makes sense it would have a number on it somewhere, even if it's nothing to do with the object's actual function of telling time.
And we don't know what's with Jhin's number but given the way the heads of the I's go it's more like a symbol of something than a number, at least from where I'm standing.
So there are reasons, doesn't mean they're all linked, since having lines in a manner similar to writing on something isn't indicative of a strict connection.
Also 4 (including Viktor it'd be 5) champions with a number on their design is not an addiction when the number of champions, including Jhin, is 129.
EDIT: I'ma take a moment here to say this: do Romans exist in the League of Legends universe? Nope. As such roman numerals also don't exist. VI and XII and X and IIII and III don't have the same meaning in Runeterra as they do for us in the real world.