Since I am always game for Mario, the restriction does not affect me. What it does affect in Kingdom Battle is how, because of Mario's mandatory slot, Rabbid Peach ended up feeling redundant over the long run because both have the same weapon, and Luigi is a somewhat better Sentry user (Sentries are basically the tiny mobile units that will track the enemy and detonates if it reaches them) if I were to use a Sentry. DK Jungle Adventure alleviated my main issue with Rabbid Peach in my opinion by enforcing her as a unit, and because of this, her qualities shined the most when put next to DK and Rabbid Cranky (e.g. her weapon has the longest range, being the only one with a shield, and the only one with a Sentry).
What I am saying is that perhaps not forcing Mario might make Rabbid Peach a viable option.
Thank you for reading.