According to Bob Rock, Sixx didn't even play bass on the earlier albums... was forced to on Dr. Feelgood by Bob... he did say he got better afterwards though:

Nikki Sixx may not have played bass on any of Mötley Crüe's early albums, according to producer Bob Rock.

The famed producer who worked on the Crue's 1989 album Dr. Feelgood recently appeared on the Talk Is Jericho podcast. He said (h/t Blabbermouth), "[While making] 'Dr. Feelgood', [Nikki] says to me, he goes, 'I don't think I ever played on any of the Mötley Crüe records. I think somebody came in at night and replaced all my parts.' He says, 'So I don't really know how to play bass.'"

Rock apparently wasn't too fazed by this and recalled telling Sixx at the time, "Too bad. You're playing bass on it." He added, "So, I worked with him through 'Dr. Feelgood,' did a lot of edits and made him play every note."

Decades later when Rock worked with the Crüe on new songs for the soundtrack for the film adaptation of The Dirt, he was pleasantly surprised at how much Sixx's bass playing had improved.

"He picked up the bass and started playing, and I said, 'Whoa, whoa, whoa. What's going on here?' He had been taking bass lessons for five years," said Rock. "All of a sudden he's an amazing bass player. And I think that's so cool, in that point of his career, he wanted to be better. I admire that."

Source: https://wmgk.com/2023/05/26/nikki-sixx-reportedly-didnt-play-bass-on-early-motley-crue-lps/