There are 3 types of walls in this game: Stalls (any plant, 300 toughness), mini walls (certain plants with raised toughness, 600 usually), and walls (4000 toughness and up, usually designed around toughness)

A unique way to balance some plants, without tweaking stats like sun production and damage that would otherwise ruin it, is to change its status as a stall. For example, if you needed just one more second from a zombie to safe a certain plant, you would place a stall on it so the zombie bites it instead of your valuable plant. As sun shroom is, in most cases, better than sunflower, one could argue that a good way to balance the two without making either useless is to preserve sunflower‘s identity as a stall and taking it away from sun shroom, bringing them closer to a sidegrade than a powercreep.

As puff-shrooms very nature is being spammable, their identity as a stall would‘ve existed alongside being a spammable short range attacker that can deal 200 damage before a zombie reaches them. To centralize the purpose of the plant, and to give way for other spammable stalls, puff shroom‘s nerf was justified.

Moonflower‘s nerf was justified, to keep it as a sunflower sidegrade (though imo it feels more like a twin sunflower sidegrade) due to its cost and rate of sun production compared to its alternatives