Coronavirus have “proofreading” functions that make them less likely to mutate meaningfully than many other kinds of viruses.

While it’s true that viruses often mutate to become less potent over time (bc that makes them less detectable and therefore more likely to spread, an evolutionary advantage), any claims that this is happening should require very strong evidence to be believed. This isn’t even weak evidence.

Most of the above is googleable. I don’t have direct citations because most of my understanding comes from the excellent This Week in Virology podcast where various virologists chat about such issues. They’re quite dismissive of the frequent “The virus is mutating” stories (though of course all viruses mutate in small, unimportant ways all the time)