Adding a new permanent character is regrettably often a measure to reawaken interest in a franchise if it is going worse (there is actually a Simpson episode parodying that fact). It may be one of the reasons why Ruby was added in the first place.
However, while there are holes in her background story, these are holes that could be filled relatively easily, unlike some of the messed up cases in some of the later sequels where no credible explanation seems to be possible (e.g. Bron's story, or the unexplained whereabouts of Cera's mum).
In any case I think they took some care to make Ruby a character not overly similar to existing characters. While some similarities are there, there are also some features not really displayed by any of the characters. For example Ruby is kind of a thinker. Now I'm not saying that the other characters were not thinking, but more often we see them (Littlefoot very much included) acting on an instinct (Ruby does so to in a few cases, but for her it seems to be less in "normal") and we rarely see any of them thinking along "philosophical" lines (while in generally this would not be something one would expect from kids some of the characters would have a lot to consider). Even for Littlefoot it would be considered somewhat out of character if suddenly he mentioned his "thinking place".
Another point that sets Ruby somewhat apart, but that is sadly never elaborated in the series, is her being neither a herbi- nor a carnivore but an omnivore who could eat both plants and meat as well as eggs (and for this reason it struck me as odd that no grownup ever showed any of the reservations towards her which Chomper repeatedly has to deal with).
I totally agree that a good background story for Ruby would be awesome, but I do not consider the lack of such a story a sign of the character herself being undeveloped.
