Yes! The RIAA certified the album as Gold in 1976, selling over 500,000 units.
Welch shared in a Q&A:
The Mystery To Me album stands out as the most ‘solid’ album, all the way through, that I did with FM. I guess you could say it was my Rumours! By that I don’t mean I don’t think that the other albums are good, just that Mystery is the one I’d take with me to a ‘desert island.’ Also, it was, for some reason, the ‘funnest’ (excuse my English) album to make.
As documented by Ultimate Classic Rock, Fleetwood said:
Mystery to Me was probably the best Fleetwood Mac album since Peter Green had left the band three years earlier. It was atmospheric and intelligent, and we knew it was going to be a big hit. We sensed we were on the verge of a breakthrough and had another real shot at the Big Time in the U.S.A.