The game figures out your role independently of whatever you were assigned in SR, so blind pick doesn't matter there (you can be assigned support but if you jungle instead, you'll be graded compared to junglers, not supports). You don't currently get ratings in ARAM.
The way you're graded is the same regardless of role/champ in so far as all the same performance metrics are considered (as in warding/kda/objectives/etc), but your actual grade is based on your percentile performance compared to all others on that champ/role. So since everything is taken into account, you need to be ahead of the curve on everything to get the best grades (and if you're below the curve, that'll hurt you). It's not that warding counts for more in the case of supports, so much as most supports do place a lot of wards, and so if you're way behind in the curve by neglecting vision you probably won't get a good grade. At the same time though, supports don't typically get many kills, so if you get handful of them on most supports, then you will be ahead of the curve and so have a positive impact on your grade if you don't neglect other areas. (Occasionally you do see posts about people with big KDAs, but they completely neglected to ward, and their stellar performance in one regard was sortof canceled out by their lackluster in another.
The champ being a factor as well as role means some supports who actually do tend to get their hands bloody (brand for example) will need more than something like a janna.
Main point being, it's all relative to how other people do in the same role/champ, versus any particular absolutes required. If everyone who played draven adc went 0/0/30 and placed 5+ wards/minute (e.g. their performances were typical of what most people expect of supports), then to get an S+ as draven adc people would not only need to ward a ton and get a ton of assists, they'd also need a handful of kills. Simply getting a bunch of kills and not warding/assisting at all would actually likely hurt them in that scenario, despite that being the 'proper' way to play draven, since it's all relative to the communities performances, and if the community was performing exceptionally in the 'wrong' ways, then that would still set the bar for him.
Of course, in the big picture every champ is played the 'right' way on average, and so for the most part if you do what you're 'supposed' to do on any given champ/role combo and you do it well, you'll get a good grade. People just tend to lose sight of just how good some of the best performances are in some roles (particularly supports), and so when their good (but not actually that amazing in the big picture) performance doesn't get them an S+, they tend to grumble that the only way to get good grades is to play the champion wrong (e.g. KSing as a support). While more kills than average would help sure, it's not because they 'need' kills, it's more likely that they're not as amazing as they think they are in terms of assists/warding and being ahead of the curve in kills can counter balancce their only average performances in other areas.
