It's not as simple as "copying is good" or "copying is bad."
Mimicking another artist
can be good for an art student's education. Many major museums and schools offer copyist programs for developing artists, and traditional European painting schools still require students to copy the works of the Renaissance masters as part of the curriculum. The intent is to teach the students the technical aspects of working with color and paint, particularly oils. By copying a masterwork, you're forced to pay attention to minutiae such as the way the brushstrokes help to establish form, the fact that colors aren't flat fields but mixes of several shades, and the way the artist establishes focus in the piece by increasing or omitting detail. If done correctly, copying masterwork teaches universally useful lessons that can be applied to original work later, the way all art education does.
However.
The majority of the time I hear about "copying" on the internet, it's not a technically intense dissection of a masterwork. It's someone deciding to solve a very specific issue with their work... "How do I draw something cute? How do I draw noses? How do I draw robots?" etc.... by simply imitating the work of another artist.
That's not good.
An artist with the correct attitude towards copying would'nt attempt to remedy deficits in their work by imitating another artist.
A sufficiently skilled artist, if they've been educated (or have educated themselves) correctly, is able to draw anything. They have the basic tool set... perspective, light and shadow, color theory, anatomy, drapery... that can be applied to any task. They can draw you a kitten, or a V-8 engine, or what they think a Martian would look like; it may not be the best kitten, engine, or Martian you've ever seen, but they could manage it, because they would know where to start.
When you're not copying as an educational exercise, but because that's how you solve problems and patch the cracks in your competence, you're cheating yourself and stunting your own growth as an artist.
Have you ever seen a picture that seemed oddly schizophrenic? A beautifully-drawn human figure, sitting in a poorly-rendered bedroom with terrible perspective? or an amazingly-well-drawn car, but the human figure leaning on it looks like a child drew it? That's where stubborn copying gets you. You never learn the basics, you just learn the surface.