No a bobber has the opposite effect of weight.

No weight limit on the flies. The Commissioner's Declaration states ..."using a single fly, or an artificial lure with a single free swinging hook. Additional weight may not be added to the line.

As long as you are using a single fly with a single free swinging hook and the fly design normally incorporates lead into the pattern, I don't interpret that as adding weight "to the line". My interpretation is that if you used lead weights, like small split shot, added to the line it would be illegal. If the normally accepted practice for a specific fly pattern is to incorporate lead foil or thread into the fly, I believe it would be acceptable.