Yeah the premice of "using art as a base" in itself is a sliding scale of morality from totally ok to WTF are you doing.
On the "totally ok end": tracing your own art for practice, tracing someone else's style to work out how to incorporate elements into your own (not the whole thing) for practice, tracing pose photos you took/are free to use/you payed the licence for, tracing someone else's art for practice and giving explicit credit to them every time you post it anywhere (and that artist is fine with it).
Iffy: tracing your own commissionned work to make another comissionned work (without specifying it's a YCH type-deal/clearing it with both commissionners first and ajusting prices accordingly), tracing stuff people have explicitly said not to trace but it stays only as practice and you never post it.
Very iffy: tracing someone else's art for commissions and specifying it's traced
Illegal: tracing someone else's art for commissions without specifying it's traced beforehand/lying saying it's not traces at all (false advertising, a commission is generally understood to be a unique original piece of art made to the commissionners specifications, tracing takes out the original part)
Here it's very much more on the very iffy side of things and the commissionners at least should be made aware.