I'm kinda curious, how do you define a character in fiction "average" or "developed"?
Change over time. If you're developing a building, you're using a basic structure and adding onto it in various ways until the end product becomes more than that basic structure. In a similar way, developing a character means that they start off with a basic idea or impression yet end up different in the end through changing experiences. If a character goes from Point A (the start of an anime, let's say) to Point B (its end) and largely remains the same character, then that's what I would consider "average," though I'd rather use undeveloped instead.
On that note, I think the issue with iM@S as far as character development (and reflection on that) goes is that the various media are NOT going to be consistent. Let's look at Haruka. Haruka in the games is not entirely the same as Haruka in the anime or even Haruka in one of the manga or novels. That's not really a bad thing, since there may be certain works that can show a different side to the character, but I think there are clear basic ideas of Haruka as she is that can be traced throughout. You have basic things like her tripping and her baking skills or her passion for singing and becoming an idol. Yet certain other traits she has shown, from the extent of her kindness to her personal insecurities to even small quirks that come around in certain situations, are not going to be reflected the same way all the time. Like the anime presents her personality and insecurities around her concern for the other idols being together like she had hoped, while TWIAO plays off how both Haruka and Producer think and act similarly to show how she internalizes some of her concerns and tries to push herself too hard at times. This may be where personal taste comes into play, but I think an effective portrayal of any character, not just Haruka, is being able to establish those basics and then present a challenge to them AND make them convincing.
This can go on further, but all I wanted to point out was that the same character can be seen in different ways, and it's up to the skills and attention of the writer to convince us that the interpretation is valid. Different strokes is a thing for sure, since what attracts you isn't guaranteed to attract me, but I guess that's what discussion is for.
Character forumssssss 
I also don't feel like not having an interesting backstory makes for a weak character.
Also this. A backstory is a means, not an end. Its potential to help make a character "interesting" can backfire just as badly as it could succeed, and not every character needs one.