I blame both, to be honest. I blame BioWare for the obvious creative fuck ups, because they're most responsible for those. Crappy writing is crappy writing, and thus the writers (or writer) will be the target of criticism. I also loath the Walters and Hudson's decision to annex themselves from the team to write the ending which is, ultimately, a decision of BioWare.
But I don't think, looking at the big picture, EA didn't play a part in all of this. For example, I'm fairly sure Chobot's presence in the game, relative to other weirdly neglected assets (eg: Tali's face), is a consequence of EA's marketing team. EA obviously had IGN as their big marketing bed buddy (see: IGN's wiki and review layout), and I would not be surprised if Chobot was marketing's bold idea to get a familiar face in the game.
Same goes, to a lesser extent, Vega. EA were the ones pushing for ME3 to be an 'entry point to the series. This was their big marketing bullet point. It's also the one big thing I figure impacted the creativity at BioWare the most, EA pressuring the team the change or add components that fit EA's master plan. Vega was apparently written as a character supposed to reflect what a new Mass Effect player would be like.
How reluctant or welcoming BioWare were to EA's suggestions is impossible to know without being a fly on the wall during development. Same goes for the extent of these suggestions and just how, exactly, they influenced the game. But I do think it played a part.
As for the ending catastrophe, I blame BioWare's writers responsible. It was a creative decision, and a bad one. But I also suspect that had the team been given more time to work on the game, we would have got something else, or at the very least, something bigger.