Not really. It's not that hard to make a tiny incision on the packet, dispose the contents and add tiny powdery stuff back in and seal it. A real plot hole would be Lydia drinking the coffee/whatever the fuck she drinks till the last sip and not realizing that it tasted funny.
But then, Walt probably did mix some stevia with ricin to not raise any suspicion.
Does the winkie really not give away the fact that comment also is tongue in cheek?
I DO NOT THINK THAT IS A PLOT HOLE, PEOPLE. That is called a joke. Apparently it's not a very good or obvious one.