I vote nay, since it feels like a hacky solution.
The annoyance depends on the sig, tbh.
Anyway, I have other ideas:
1) When someone puts a quote in their sig (i.e. a quote to a post on this forum), or puts too much text, it sometimes looks like part of the post. Perhaps changing the style somehow will make it not match the other text. Make the sig text smaller or duller?
2) The size of pics in sigs should be limited. And sorry to be such a spoilsport, but animated gifs are annoying too. I click "Block Element" when I come across a truly annoying sig.