It's not always about credible. Sometimes it's just about entertaining and funny -- which, IMO, Teschmacher and Otis were in Donner's classic.
Anyway, there's absolutely no reason Gunn couldn't be giving us a more serious take on Teschmacher. A lot of Superman characters are significantly reimagined and repurposed from production to production. Supergirl's Eve, in fact, proved to have very little in common with Perrine's character.
Teschmacher, as Luthor's trophy wife, who is much more intelligent and devious than she lets on would be a good character.