I think this depends on the perception and the attitude with which this is said. At least in my experience, the word is very often used by people in informal situations, without the negative connotation Miki8307 mentioned. For example, if I give to a colleague something from the printer, I would get a ”mersi”, not necessarily a full ”mulțumesc” and this is ok.<h1>my2cents</h1>
Miki8307 is right until a point. Mersi can be used sometimes with the sense of ”Thanks for nothing”, but you are right AnaTechWr, only the tone and attitude of the speaker can made this word to sound impolite. Let's add that ”Mulțumesc” never may sound impolite.
Thank you, AnaTechWr
Yes, but the roots of ”merçi” in Romanian are not so old. See this article https://www.historia.ro/sectiune/general/articol/cate-cuvinte-franceze-are-limba-romana. Sorry of not having a good English article, but you can translate this one with a tool.