I don't think Capshaw is the problem - she's playing the character as written. The tone of the entire film is cartoonish. Would anyone else have been much better with the same material? Clearly, they were trying to make her the anti-Marion by having her be a spoiled, helpless, girlie-girl and took it too far.
She's also playing the character as directed, so a large part of the "blame" lies with Mr. Spielberg. It's a completely annoying performance, with no redeeming qualities whatsoever IMO.