Sherilyn Fenn was super charming in TP, I agree.
Great series even though many of the actors were average at best, but in the Lynchian world it works fine anyway.
They were asked to play as if they were in a soap opera I think (it's quite obvious, if you analyze their dialogues and their situations), it's one of the many aspects of Twin Peaks. There's supernatural horror, surrealism, soap opera, black humour, melodrama, ... It's actually impossible to classify the series, genre-wise.
And the actors portraying characters like Bob (which was not an actor to begin with - he's part of the many improvised situations of the series, I read), Andy Brennan, Lucy Moran, Josie Packard or Catherine Martell, and some others were chosen and given these roles for the needs of the series, to bring this melodramatic/soap-operatic/campy style.
Some like MacLachlan (as Dale Cooper) or Ray Wise (Leland Palmer) were absolutely outstanding, in a more academic way.