While playfield and mechanical design factor into the fun of the game it is code that brings it all together. LoTR is an excellent game despite the clunky playfield, the Gollum shot that consistently rips you off, the weak upper playfield, the DTR mode that hardly worked on location (as well as the trouble that many games had with the sword lock). Why? Because of the code. The rules, along with the use of quotes and music get the player to overlook the subpar flippers that can't handle a long game, the stern slings and pop bumpers, the Palantir target that doesn't always register, and the left ramp U-turn that takes it's time giving you the ball back on a missed shot (almost as bad as MB), etc.
So attributing the games only to the designer is shortsighted, I think.
I enjoy a great layout, but without good code the game comes up short (ie TOM, ToTAN, RS).
But with nice code, even a simple layout becomes very enjoyable (DW, JB, AFM).

So, for me, clever code easily trumps silky smooth ramps and loops and the feel of a satisfying shot.

I disagree. TSPP is very fun. Keeping the timer going is way addictive. Victory laps on all shots for 5M each is extremely satisfying, as is completing AI. Also lots of fun: getting big double super JPs, getting the game to give you an EB from crazy EB, stacking I&S MB with couch MB, pretzel MB, stacking hurryups, and much, much more. So much to do, such creative and clever rules.
This game proved to me that Stern could make an enjoyable game, and the rules set a new standard in pin. Stern suffered a great loss when they let KJ go. Fortunately for them, they still have LFS there, so they still have the potential to make games with creative rules. But if Lyman is not on the game, look for mostly rehashed, uninspired rules from Stern.

"TSPP has excellent code and ruleset but really isn't all that fun. The game layout is super unique also. So the best code can't make any game fun."

This does not make sense. The purpose of pinball code and rule sets is to make the game fun - by setting up goals that players want to achieve, having appropriate scoring, keeping the players' interest, etc.
Provided that the rest of the game is functional, if a game has excellent code, then it will be fun to play. If the game is not fun, then it does not have excellent code.

Why do you think that TSPP is not fun? And why do you think that the code and rule set are excellent?

"By your logic, I could have a whole game playfield be say just the lower playfield of acdc and with great ruleset, the game will be fun?"

If a programmer managed to take those few shots and make a ruleset that is "great", then yes, the game would be fun. For many players that I have talked to, code is more important than any other aspect of the game.

"The ruleset is deep and has massive stacking. I like having to keep the timer reset and preventing the modes from stopping. In the case of TSPP, its issues are all in the physical layout. I don't enjoy the mechanics of the game. The shots are not satisfying nor fun. There is a lot of stop action, ball times are way long, the upper playfield shots feel like a chore. I don't like the mini ramp in the garage where the ball often ends up in the pop bumpers. In short, I don't have any fun playing the game due to the layout and in spite of the decent code. So no, just having great code does not make a game fun."

The game does play long on factory outlane settings, but this can be adjusted so that the game plays much shorter. So you like to keep the timer going, but "don't have any fun playing the game"?
If I like the rules in a game, then the game is fun for me to play. Indeed the amount of fun that I have on a game is mostly proportional to how much that I enjoy the rules.

I enjoy playing PinBot, it has nice music and nostalgic value for me. But there is not too much to do, so I would not play it for hours a day.
Now JB, with the same playfield, does not get old quickly at all. It is a very fun game to play. LED did an outstanding job with this game (as usual) and the rules are fantastic. Casino run, super jackbots, mega visor, cheating - everything that is there works well.
So JB, with the exact same shots as PB, is a much more fun pin. JB doesn't have deep rules, but what is there is very well done, taking the classic playfield and adding fun goals.