You might get the same reading- but you might not. Krausen would coat the hydrometer, making it hard to read as well as a bit heavier so it would have to be removed, cleaned, sanitized, and put in the beer.
It's easier to just use a turkey baster and pull out a sample that it would be to do all that. Plus, hydrometers are fragile! During active fermentation, it would get knocked all around (have you ever seen the activity during fermentation- it's wild!) and possibly break if it bumps the side of the fermenter.