If you are planning to sell your game, no, using stock art isn't "cheating"... however, if possible, it's better to have original content merely be supplemented with stock art. If your game has assets that players recognize, it can leave a bad taste in their mouth.

This includes: Music they recognize ("Hey, they stole that from X!"), sound effects ("Pfft, that's cheap, they just used stock sounds!"), and art ("Bah, this is the tenth game I've seen this month that uses RPG Maker tilesets. The game is probably junk.").

There was an indie game recently that got heavily lambasted [warning: language] for using mostly purchased stock 3D models (and further lambasted for his terrible PR response). People comment frequently on RPG Maker artwork, and I've personally passed on some games that use those tilesets. Other games have been called out for using stock music - this isn't a big "crime" if, again, the stock music supplements the original music.

Yes, a game can be fantastic and purely use stock assets, and by judging the game by it's appearance may leave me missing out on experience what could be a fantastic game (and occasionally, there are fantastic RPG Maker games), however, in an age where there are more games than we have time to play, we have to filter them somehow. Stock art leaves an impression that the developer cheaped out ("Why would I pay real money for a duck-taped hodgepodge of assets that don't really fit perfectly together?"), or that the developer was lazy or in a hurry ("And what else may he have cheaped out on? Is the actual gameplay going to be buggy, rushed, and unpolished?"), or that it's his first game and he's inexperienced ("Why would I buy an inexperienced and probably broken game?").

So you have to be careful not to give your potential customers a ready-made excuse not to buy your game.

For myself, for the commercial indie game I'm developing, I'll be using alot of stock music to supplement my game's original soundtrack (a friend offered, and it came out decently enough). For artwork, 99.99% of it is made by me, but I do incorporate a few public-domain pieces (mainly portraits on walls, but also a few textures). For sound effects, it'll probably be mostly public domain or stock assets - but I'll have to be very careful to make sure it sounds consistent, is decent quality, and isn't recognizable.

What is your intent?

Are you cheating yourself because your initial resolve was to make a game on your end? If so, yes, this is cheating.

Are you partaking in a game jam that requires all content to be made by you? If so, yes, this is cheating.

In possibly all other circumstances I can think of, this is NOT cheating in anyway.

These assets were made by somebody else.

From a personal standpoint however, I try to hire freelancers to do custom art as this insures this art is never used in another game. I hate it when that happens, but otherwise, its not cheating than to require help from others.

Not really, as you can always replace the bought assets with newer ones later down the road.

However, there can be a practical issue with stock art. And that is - it may look good but it may not necessarily be game ready. 2D sprites are, from my experience, almost always game ready for game engines such as GameSalad and GameMaker (so most likely you don't need to worry about practicality). Just import, drag and drop. The practicality drops immensely with 3D assets, because modern 3D game engines require UV maps, lightmap channels and a whole lot of requirements so the assets look and behave well in the engine. I unfortunately had to bin good looking medieval houses from OpenGameArt because they had a whole lot of issues and they don't like UE4 for some reason. I found it easier to follow a Blender tutorial and do my tweaks myself than use stock art and fiddle with the tons of issues they may have.