Yes and yes, but it's not always the case obviously. You need decent tanks to create space to allow DPS to operate in. Team composition, hero choice, and maps can also affect performance.
Of course, but it seems like there are a lot more competent tank and support players than DPS. It just feels like most of my comp matches are decided by one team having better DPS than the other. And I can't even get mad at bad DPS, because I know I would also suck, which makes it all the more frustrating. This is at platinum, by the way, but I'd imagine it's not much different at other ranks.