If your goal is simply to win chess games, then yes; you should always play like a grandmaster (to the best of your abilities) even against less skilled opponents
(again, this is if your goal is simply to win. Really putting the squeeze on a novice is a great way to make that person lose interest in chess).
You said against novices that :
You don't need to waste moves defending your army because your opponent probably won't even notice the hanging pieces.
To this I say a few things. First of all, a chess strategy that assumes "your opponent probably won't notice" is pretty flawed. While you may have a high chance of success against novices, people tend to develop the strategies they use the most often. If you repeatedly leave hanging pieces, you'll slowly find yourself checking for hanging pieces less often.
In addition, if you've misjudged the quality of your opponent and blunder away a free knight early, you've begun that game down on material. Blundering a piece away isn't so bad when you're up substantial material, but that's a decision made on the quality of your opponent's position, not the quality of your opponent.
Lastly, again, "people tend to develop the strategies they use the most often." The more games you play sub-optimally, the fewer games you play optimally. I know that's obvious, but you have to consider that in the long term, your skills won't be developed if you avoid using them.
You can checkmate your opponent faster by doing things that're usually considered bad strategies, such as moving your queen out too early in the game.
That's potentially true, but the goal of the game isn't to checkmate your opponent as quickly as possible. With the exception of a few notable games, it's not important if you beat your opponent in 4 moves or 100, so why risk the game? For example, you could try Scholar's mate against a novice and probably win most of the time. However, if your opponent is familiar with this, now you've played a sub-optimal opening for pretty much no reason.
Additionally, I discovered in the Chess.com mobile app that its "best move" hints change relative to the computer opponent's level.
This is likely because chess engines view time as a precious resource. When you ask for the "best" move and the computer knows it's playing a novice, it's not going to delve as deep into lines, because it doesn't have to. This does undercut my answer a little, but keep in mind computer play and human play is very different. A computer has a harder time allocating time for future moves, and so you'll find it trying to shave precious seconds off where possible in case it needs them later.
A more interesting question though, is Should I make the best move available every time?
Interestingly, the answer to that question is no. That's because chess is a strategy game, and a large part of strategy is evaluating your opponent. When outmatched, you'll sometimes see grandmasters start with a traditionally weaker opening. This is because they've prepared it; they've analyzed most of the opening lines, and they know their opponent probably hasn't. As such, they might be able to leverage a small advantage that's less obvious to their opponent right away.
In addition, there's some interesting psychology behind making sub-par moves. In particular, Mikhail Tal was notorious for deliberately ignoring the best move available. However, he had the skill required to back up these moves, so players quickly became very cautious when he gave them an "obvious" response. That often led to them making their own sub-par moves.
Tal's style of play was so intimidating that James Eade listed Tal as one of the three players contemporaries were most afraid of playing against (the others being Capablanca and Fischer). However, while Capablanca and Fischer were feared because of their extreme technical skill, Tal was feared because of the possibility of being on the wrong side of a soon-to-be-famous brilliancy.
There are plenty of stories of Tal making "bad" moves which confused or concerned his opponents enough to give him an advantage, but that's similar to the "weaker opening" point as well; Tal knew his advantages weren't strictly based in material.
When evaluating their own play, your opponents will likely assume you'll make optimal moves. If you do, they'll have a line prepared and it's truly a battle of who has better chess strategy. But when you throw in a few moves they didn't expect, they have to reevaluate their own line and they have to try and understand what you're thinking. Tal was an expert at this; his habit of making unexpected moves made it harder for his opponents to evaluate deep lines. If done correctly, this can give you a larger advantage than the material or positioning you've offered.