Donna Douglas : IMDB Picture of Donna Douglas

Was the cowgirl character in Toy Story based on her?

There is resemblance.

I doubt it very much; this appears to be a very obscure photo of Donna from early in her career (possibly appearing on a western tv show but I suspect just a publicity shot in a cowgirl hat). I don't know why they don't have a picture of her in character as her iconic Elly May Clampett as the main photo.