First of all, the "90:10" or "75:25" numbers are arbitrary by definition, there is no way to objectively quantify how hard or easy a matchup is. It is impossible.
Anyway, we are not "making a 90:10 matchup out of nothing". The matchup is as it is. If DeDeDe can infinite Luigi after 120%, then that's how it is. And you can definitely wiggle out and be perfectly safe from a running grab followup, it's not a problem.
If you think the matchup is bad, then you shouldn't play it. It is neither my concern, the BBR's, or yours to make sure that every character has a fair matchup.
If your character has a bad matchup, it's up to the player to deal with it or counterpick. That is the way that the game is, there's nothing else to it.
You need to realize that we aren't "making the matchup worse" by "allowing" the infinite. We aren't "allowing" anything, we're saying that you should play the game and work around it accordingly. If a matchup is bad, it's just bad and that's that.