I don't think they just did that so they can shoehorn in funny elements.
That's
exactly why that trope is used,
every time it's used. Nothing about her story requires that she be as dumb as a sack of hammers,
except for the cringeworthy attempts at "humour".
The things that are necessary for every single bit of Sophie-related plot to work:
- Sophie isn't an ordinary human being, and has some sort of innate magic capability.
- Sophie does not immediately understand what her exact origins or purpose are.
- Even after 'death', Sophie can eventually re-materialize herself.
The things that are in no way, shape, or form necessary for any of the game's Sophie-related plot to work:
- Sophie is dumb as a sack of hammers, and despite springing forth into the world fully formed, knowing how to walk upright on two legs, how to speak and understand perfect English, the difference between food and dirt, and how to kill people with her fists, she arbitrarily does not understand a
tiny handful of extremely simple concepts that all just so happen to be along the lines of "friendship".
- Sophie is eternally about 14 years old.
The first is done entirely so that they can make all those 'hilarious' jokes where someone says something that is in no way confusing,
and Sophie is still confused ahahahahaha, and so that they can have Very Tender Moments™ when her friends teach her how to be happy. Barf.
The second is done for the exact same reason that Lymle is arbitrarily in the body of a small child in Star Ocean 4: To fetishize little girls, because their gross otaku audience eats that shit up.
Sophie, and the other Graces characters, are certainly not the most original people you can find, but while these tropes could be considered the core, there's usually an attempt made to ensure that there's at least other tolerable qualities around that core. The same thing they usually try to do with like...all Tales characters, though how well it's done varies. It's extremely easy to see the anime tropes for each of Vesperia's characters and these tropes get shoved in your face several times over the entirety of the game, but most people don't call them SO4-tier or whatever because the overall presentation and the fact that they made some kind of attempt at depth at least brings it beyond that level.
Some (all?) SO4 characters are basically just the trope. There's nothing interesting to find about them because all they do in the game is enforce the stereotype through their animations, voices, and dialogue, or they contribute so little to the game in the main story or side events that there's nothing to find there but that trope.
That's the problem, though. The characters in Graces
are nothing but
just the trope, and nothing else. They're just
more of it. When the premise of a character is 'monotone girl who doesn't understand the world or emotions', and the "character arc" is 'the rest of the characters make friends with her and she learns about the world and how to understand emotions', that's not character development. That's just the trope in long form, rather than shorthand. And that's all that either game does; neither of them are
better at it, and Graces is scarcely even better at the
presentation of it (shit, the 'childhood friend' character in both games actually literally has the exact same English voice actor). It's a stinky fart of a story, either way, but one of them is a
little stinkier and sticks around for a minute or two, while one is slightly less stinky but is part of an evening-long gas attack that just never lets up.