Some people have suggested they enjoyed Pegg's performance. Yet others, including myself, have stated that the portrayal of Scotty was terrible.
Can a performance be good and terrible at the same time?
In the case of Scotty's portrayal: yes.
Pegg's performance was "good" in that, if you separate his scenes out from the rest of the movie, he did get cheap laughs. He acted in a silly, over-the-top way, and this always gets a chuckle.
Isn't this good, then?
No. It's not good because, even though his scenes, taken alone, may not be that bad, they do not fit in with the rest of the movie. They might be appropriate in a Disney movie for little kids (which usually have some over-the-top character for comic relief), but not here. Anyone can get cheap laughs by saying silly things loudly ("exciting"; "towel"): what's hard is to do so without detracting from the other actors.
Here are three specific problems with the portrayal of Scotty and why it harms the film:
1. Humor is too silly and obvious. Scotty is used as slapstick, as broad comic relief. He says mostly silly things, in an overacting, hammy manner. The comments about food on Delta Vega, the towel comment, and the "exciting" comment, were all attempts at humor by being silly.
But the humor in Star Trek is not supposed to be silly or slapstick. The other characters' jokes are wry and ironic: "is there a problem, officer?"; "no, not really." "at ease"; "I would cite regulation, but I know you would ignore it"; "aural sensitivity"; and so on. It's out of key, in the midst of these clever and subtle lines, to have childish, over-obvious humor.
2. Inconsistent with TOS. The other characters all have a direct and clear relationship to their characters in TOS. Much of the joy in watching the film is noting the carefully craftsmanship by which the differences and the similarities between the movie characters and their Prime versions are delineated. Unlike the others, however, Scotty is nothing like the TOS character. Scotty Prime did not spend all his time making stupid and overobvious jokes: he was in fact one of the more serious characters. He gave the cast balance. Pegg's portrayal destroys that balance.
3. Not credible. Engineers just don't act the way Pegg does, in general. Not for one moment did anyone believe Pegg was actually an engineer: he was acting like a salesman or class clown who happened to be cast as an engineer.
So, the key problem with how Scotty is portrayed is not that the performance is not that it was bad per se; it was that it was out of place in this movie. It just did not fit in with the tone of the events of the movie; with Scotty's character from canon; and with how engineers might be expected to behave. Every scene he is in takes the viewer out of the movie, makes the movie seem that much less real.
And one other key thing: we lose the true character of Scotty, who was a great character on TOS. And we replace this great character with a kind of Jar Jar character, with puerility and nonsense.
