I think it's negative effects are already seen. Paper Mario suffers from similar looking NPCs that tend to also act the same bdcause they're based on the regular, childlike Toads. Aside from the major characters barely any NPCs are notable unlike in the old games. Their designs were reflections of their character. Toad Town is a good example, you see what the characters are like before talking to them.

Even gameplaywise I think it has negative effects. Enemies have way less variety in atracks and design, look at something like Partners in Time. That game had Piranha Plant Planets, Koopa Chameleons, Thwomp Stacks, alien Wigglers, cool enemies with new attacks and designs that would be hard to replicate under the limitations.

In general the modern Paper Mario games have very similar enemies, the generic NSMB style. Enemies have variations like wheels of paper stacks or attacking using tools but those are mostly the shy guys, and there are so many Shy Guys in Color Splash. It's boring to fight them. New enemies or actual subspecies help give variety.

It feels like a weird excuse as well.

So Dream Team had rule breaking characters from previous games. Assuming Sticker Star was made with the mandate, that means that old characters could return, unless this is just a Paper Mario thing, yet this was never taken advantage of in new Paper Mario games, which would be an easy way to slightly appeal to older fans.

Nobody actually likes these mandates. The people who get annoyed at those complaining about them probably don't care either way, nobody complained about the Toads in Paper Mario, the mandates just make games slightly worse and less immersive for no benefit.

I actually question their effects outside of Paper Mario because other games like 3D Mario games, the sports titles, etc. seem to be immune to this.

Also what's with the Mural Koopas and Goombas?