The smartest choice of the movie was making it more of a comedy than a horror film. We knew from the trailer that the protagonist was functionally immortal. This killed any sense of suspense throughout the film, so trying to make it scarier was never going to work. However, they should have taken this farther. As it was, Happy Death Day couldn't quite decide whether it wanted you laughing or screaming.They probably should have made it a bit funnier and done a better job marketing the funny side of the film. And a lot of horror fans could have come away disappointed by the lack of good scares.
For a mystery, it was pretty predictable. I made a suspect list on the first time loop. Number one on my list was the roommate. (I was right.) Resting bitch face was a big clue. The other clue was the immediate attitude she gave Tree when she entered the room but then ended the conversation with sugar, both literal and figurative. For the record, suspect two was the dad. (Posited theory: He also killed mom and was coming back for his daughter. Evidence: Father/daughter estrangement and the amount of anger about her skipping dinner.) This would have been a much darker film. Suspect three was Carter, the guy she woke up with. (No real reason. Just sets up a potential twist.) Every suspect Tree listed was an obvious decoy.
The movie did fool me at one point. In the fourth act, Tree decides that the serial killer is her killer. Throughout the movie, we see that the Babyface is slightly larger than Tree and not very muscular or agile. The serial killer was noticeably more capable at killing. Babyface was also focused on keeping her identity concealed, while the serial killer did not care. The biggest clue to this decoy was that Babyface clearly knew her personally, but I thought the movie might be dumber than I was giving it credit for and I fell for the eating the cupcake fake happy ending. This twist and the ultimate revel of the killer was the best part of the movie.
It's a little unrealistic that Tree took so many loops to beat Babyface. After all, as noted above, Babyface wasn't that physically intimidating or skilled and Tree had the advantage of foresight. This could be explained if Babyface was also looping through time, especially if she was somehow causing the loops. It would also explain the seemingly supernatural nature of some of her kills. But, by the end, they make it clear that Tree is the only one looping through time.
I liked the feminist prospective to the film. The damsel wasn't in distress. She went full on commando. Whenever a guy was in a position to potentially help, they were quickly offed. Tree once even sacrifices herself to save Carter. Carter is possibly the only prize boy I have ever seen in a movie, as I can think of nothing he ever does to earn Tree's affection other than not being a rapist or a thief. (Or at least he lied about not being a rapist. He did apparently remove her pants.) There was an overt jab a campus rape culture and the movie passed the Bachdel Test.
Why did Bayfield college pick the baby as its mascot? What college would chose a mascot that does nothing but cry and shit their pants? Are all of the other B nouns taken? These questions were extremely distracting and totally took me out of the movie at times. Presumably, the filmmakers picked the creepy baby mask first and then needed some reason why other characters would have the same mask to set up some fake outs, but the mask makes no sense in universe.
Going in, I was not looking forward to listing to that annoying 50 cent song from the trailer on repeat. Unfortunately, they used even more annoying jingle to be the I Got You Babe of this Groundhog Day. Presumably because they could not afford the royalties to play In Da Club on repeat in the movie. (Apparently, Happy Birthday To You is public domain now and Warner Brothers spent years collecting millions of dollars on an invalid copyright claim. I learned something while trying to find out how much they paid up for the music box gag.) Tree is avoiding her birthday and her dad because of the painful memories they bring up about her mom, so illogically she changed her father's ringtone on her phone to "It's your birthday. You know you gotta pick up the phone." (Also, when? When did she have a chance to do this?)