As far as I'm concerned there's just 2 main criteria you need to keep in mind in your approach to art.
- What you do needs to be legal.
- The result should be as intended.
Number one is undebatable. If you steal images or materials that don't explicitly state you are free to use them in commercial or non-commercial works (whichever one you are doing), then you're in the wrong.
Second one is more personal. It is entirely up to you whether or not you have fulfilled your goal here. If you do art only for yourself, you only need to bear your own opinion in mind. If you also want to gain an audience, you need to bear general opinion (or at least the opinion of your target audience) in mind. If you aim to be hired in the industry, you need to bear the employers opinion in mind. Reason for all this is that, as stated, the result should be as intended. If your intended result is to make money off of your art, then your private opinion of what looks good enough is no longer applicable. If you are too much of a perfectionist rendering you too slow for the business, you have failed your second mission. If you are fast but your art is not up to par with the business standard, rendering you incapable of supporting yourself financially, you have failed your second mission.
By this very definition, whether some of y'all like it or not, many of those webcomics that "mess up using 3d backgrounds" are still very much fulfilling their goals as artists. They are keeping it legal (assuming they paid for the full professional license of the program they use for the backgrounds) and they meet their intended result. Anything beyond all that is just your personal opinion, which you are very much entitled to but will never make them any less accomplished or professional as artists.