G2 had a good base for characters to work with. Ryudo was a dick of a main who was actually fun about it, and Elena's naivete bounced off it nicely. Roan was a kid who wasn't annoying for once, and Millenia was Millenia. Issue is...as the game goes on, characters start losing these traits. The Melfice arc completely degrades Ryudo's character; he no longer has his snappy lines and he's being totally serious. I'll grant that you could argue its reasonable HERE, but he doesn't have the rebound you'd expect (as in, the shit is over, now time to get over it and get back to his usual self); he instead DEGRADES FURTHER. ANd thing is, G2's core plot itself was NEVER good; its basically "Run around the world beating up parts of the devil!" Pretty bland, though it had decent ARCS, the core plot was always bad, and Melfice didn't do much but add to Ryudo backstory, at the cost of hacking away at what made his character actually fun.
G1...I dunno; I wasn't fond of the cast from the get go. Seeing Justin get tossed out of a building from the start, and then get repremanded...that made me go "...this is going to be a long game" instead of "Oh how cute!" Justin was unbelievably stupid about things. The worst part about Grandia, as I've stated before, is that its not just Justin is an idiot...
His idiocy gets PRAISED by everyone and everything barring like one or two fo the villains and Lily. He says something completely naive and stupid, and then game is like "Oh! THat's so insightful and spiritual! To be young again!" Worse yet, Feena has quite possibly the quickest slam of Chickification ever. Here we have the self proclaimed World's Greatest Adventurer, and it seems based on her previous actions, people actually accept that she is this, and she even shows she's got experience at this stuff despite her age...
THen she meets JUstin, and after one dungeon with him, she's already turned into a Damsel in Distress, and the rest of the game is just bawling over Justin. Now it is possible to make a character like that work...but you need to be gradual about it. Look at FF6's Celes if you want a similar style of development, for someone who actually pulled this off successfully (not perfect comparison, but best one that comes to mind.)
There's a difference between "Light Hearted" and "downright bad writing." Grandia falls under the latter. A game like La Pucelle is Light hearted, while being overall serious, but it doesn't just sort of spit on its characters. And I refuse to give credit to Grandia for being parodies, cause well, they fail at that too. Again, Feena's moment chickification isn't a parody, its a face palm inducing moment.