I think it's less that the later suits were silly, and more that they create some world building and writing issues.
The mk 3 was implausible, but it was at its core understandable. It had lots of really powerful servos, armor plating, missiles, etc, stuff that even if it doesn't exist IRL, possibly could. The only crazy stuff were the repulsors and the reactor, and in iron man 2 he said those were only about ten years ahead of the rest of the world. Ok, fine. I can buy that.
The issue with IW is that it takes the Homecoming Vulture vs FBI scene and makes it even worse. If you'll recall , the Vulture had a super tech flying suit and a ray gun powered by alien technology. The feds had glocks. That scene pretty well underscored the MCU's big issue, that at its core it's still just the regular old real world with comic box stuff stapled onto it, and those two sides don't interact. So Tony is running around with nanomachines and super tech bullshit, the rest of the world is just modern earth, and those things don't fit.
The other issue I have is that now that Tony can just literally pull weapons and gear out of his ass, it removes a lot of his limits and let's the writers just do whatever with him, instead of having him actually work to overcome his enemies by using what he has intelligently (like he did in the first three movies). Early iron man had to worry about ammunition, had to make design tradeoffs to fit the weapons he wanted in his suit, had to make sacrifices if he wanted some highly specialized gizmo, had to be clever when fighting someone who was resistant to his normal weapons. And then in later films he's just going "huh, Thanos can only use the infinity gauntlet if he makes a fist, time to use my new infinity-glove-restraint-anti-fist-closing-inator and stop him from using it".