Ironically enough, this episode is consistently ranked lowest of all the Black Mirror episodes. AV Club gave it a C+ rating (their lowest to any ep in the series).
Some excerpts from their review that make me want to cry! (reordered for dramatic effect)
We watch as Waldo enters the by-election, largely as a joke...
There’s just not enough there to suggest that Waldo’s moment would last much longer than 15 minutes, and the show doesn’t help by having all of its characters agree. His political message is a kind of “fuck ’em all” apathy. As Gwendolyn shouts at Jamie late in the episode, if he were calling for revolution, that’d be something. But he’s just calling all the politicians venal liars. Anyone can do that; it happens all the time.
It’s especially strange that everyone acknowledges Waldo’s unfunniness. None of the politicians like him...
... Waldo has a nastier, more violent streak to him, and incites crowds into throwing shoes and beating people up. This is even harder to believe, especially in the real-world atmosphere the episode creates...
There’s a few fundamental problems with the episode. The major one is that Waldo isn’t funny, and he rarely even makes the kind of cogent points I expected him to make. He didn’t need to be funny, but outside of one particularly successful rant, his content is entirely dumb dick jokes and profuse swearing, which would certainly attract some media attention, but probably not the kind of phenomenal success he experiences in the episode.
Nonetheless, Waldo is a hit...
The episode hasn’t remotely succeeded in getting us to buy that kind of radical shift.
It’s just not much fun to watch, and the episode is not nearly as clever as it thinks it is. I give Charlie Brooker a lot of credit because he’s very smart, but I just couldn’t detect any real brilliance at work in “The Waldo Moment.”
Really, the biggest criticism of this episode should have been that Waldo didn't win the election.