Roger Smith said:
toads are just these filler NPC's that have little to no personality
toadette is basically just a toad they decided to make female
princess peach is a stereotypical "oh no I got kidnapped help pls tyvm" character (outside of RPG's)
bowser is also a stereotypical "dur hurr I'm evil and want to *bleep* stuff up" character (again outside of RPG's, but I'm a little less harsh on him since he looks awesome)
the koopalings are just glorified koopas. they legit have nothing to differentiate themselves with besides appearance
Now, I think there are considerations to be made.
As a whole, I would say that Nintendo so far failed to recognize the importance of characters, location and story develoment in the Super Mario series.
Koizumi and his team got these aspects pretty much right in Super Mario Sunshine and in Super Mario Galaxy and - guess what? - Rosalina became a fan favorite, Isle Delfino became a memorable place in the Mario universe, with his inhabitants still used even in the
more recent games not even developed by Nintendo, Bowser Jr. immediately became an important character - pretty much a year after his introduction - and so on.
It is true that gameplay must take the front seat, and I will never deny this, however this infrastructure of well developed characters and locations is something tangential to it - it doesn't necessarily negatively affect the gameplay if done right - and if not abused with everything being constantly reinvented, it can build an interesting universe full of recognizable locations and characters you might like or not, but that still contribute in creating a really interesting fresco - Super Mario 3D World had a huge chance in this sense with the Sprixie Kingdom, the Sprixie Princesses, Peach not being kidnapped and Mario and Luigi being plumbers at the beginning of the game, but I don't think we had many memorable places and characters out of it, unfortunately.
To name another peculiar example, look at
Nabbit's page on the Wiki, where his quote comes from a game developed by SEGA - this would have been quite grotesque 20 years ago!
That being said, I don't fully understand your point: while I agree that original RPG characters are really nice and usually have had their own personality and barring the Toads that are not a single character (and actually, even in Captain Toad: Treasure Tracker there are like 5 of them - little more if you count the Mummy-Mes, this "Toad saturation" seems to be mainly a "new Paper Mario" thing), I would say that all the characters you listed had the personality developed beyond what you said, in RPG games (including the Koopalings that badly needed it - until then most of the in-game lines of dialogue came from the PC version of Mario is Missing plus
an obscure "interactive" Japanese game, while most of "character development" came from Mario Kart 8 ) and spin-off games, including the Mario sports games - and even if nowadays there are less of them, the handheld versions, and now even the Wii U version of Mario & Sonic games have some lines of dialogue that show us the personality of Team Mario characters, and Bowser's story in the additional chapters of the 3DS version of Mario & Sonic at the London 2012 Olympic Games showed us quite an interesting Bowser, in my opinion. Actually, even Super Mario Sunshine showed us a different side of Bowser's personality.
Personally, I would be interested in Nintendo reevaluating Koizumi's works and trying again what he tried in Super Mario Sunshine and Super Mario Galaxy without the gameplay taking the back seat - it doesn't need to, as they are different aspects.
