Was Liza Minnelli a bit player in The Odd Couple?

Not so far as I can see.

The credit isn't listed on Liza Minnelli's IMDb page:
http://www.imdb.com/name/nm0591485/reference

So not likely.

If that isn't her (in red fringe, last dancer seen in Metropole Club scene) it's her double! But it's not referred to anywhere, so... Glad I'm not the only one!

I think she was twice. Once in a yellow coat as Lemmon enters the club and as a go go dancer in red fringe. It sure looks like her.

Yes, that is Liza as the final dancer we see on the far right as Jack Lemmon as Felix approaches the bar, as well as in the rolling pan left to right as Felix leaves the Go Go club after injuring his neck tossing back a shot of liquor!

In a clear continuity error, whether to have been the same character immaterial to the point, Liza almost literally runs into Jack/Felix as he first enters the club, since she’s departing wearing a yellow coat, turning to her left while departing!