Every class is "OP" in one way or another, its how you deal with their outstanding features that determines if you win or loose.
The scientists are overpowered in close quarters, peashooters are overpowered in gardens/gnome bomb because of their chilibean nukes, cacti are overpowered because they're the only class that has any kind of range and accuracy, sunflowers are overpowered because they can chain heal beams together and heal allies though walls, All-stars are overpowered because they can boot imps across the map from relative safety now and have high damage/low cooldown weapons, foot soldiers are overpowered because they can hide on rooftops and pick off players from relative safety, chompers are overpowered because they can camp underground and insta-kill unsuspecting players with a relatively low cool-down and almost all engineer classes are "OP" because of their splash damage.
Sure some variants stand out above the rest such as the chemist, toxic pea, camo cactus, etc but its these differences that make them fun to play, if they all did similar damage and really didn't stand out, this game would get dull fast. The plumber just happens to have massive splash damage as his gimmick. Yes, he's easier to use than some of the others but he's hardly overpowered if you have a basic sense of reasoning and can adapt to oncoming attacks. Easy solution to the plumber? Don't congregate in the garden, he's adept at clearing out crowds, that's what he does. If there's a plumber around, spread out, get closer and take him down. Most plumber users can't handle a face to face battle, that's why they're spamming shots from the top of that tower or from across that bridge.
Yes, he's more of an annoyance than others and he's potentially more devastating to unknowing players but asking for things to get nerfed without fully realizing how to deal with them beforehand is what ruins good games. If I'm getting dominated by a plumber I change classes, I fly a garlic drone over and pop him a few times, or snipe him out with the camo cactus. Figuring out how to deal with a troublesome character shows more skill and talent than formally requesting that the developers weaken it so y ou don't have to.
I do agree about that golf course map, more plant pots would be welcome. Give me a reason to plant all of these Scardy Shrooms.