We know Ron Weasley had a great-aunt named Muriel. However, this was on his mother's side, so they wouldn't share a name. If Muriel is related to Molly on her father's side, she could be Muriel Prewett, but we don't know if she is. Is anything known on the topic?
We don't know:
Her surname is unknown. It may be Prewett, but only if she is related to Molly on her father's side.
There's also no reference to her surname on Pottermore.
Surname possibly Prewett? As Molly's aunt, Muriel could be either the sister or the sister-in-law of Molly's parents. Even if Muriel is a connection through Molly's father, a Prewett, that doesn't necessarily mean that Muriel's adult surname was Prewett. The only thing that is certain is that Muriel is not a Weasley.
Heres another quote that enforces the fact that we can't be sure of her Surname
Ron's Auntie Muriel (no mention whether she's a Weasley or a Prewett, though her opinions suggest she is a Prewett) is an aged, outspoken, eccentric relative of the Weasley family.