The trouble with Thor is not so much that he is stupid, but that his is simple.
The difference is significant, in Loki's opinion, though the results are the same. It isn't that Thor can't understand complexities. Loki is quite certain his brother has more than enough brainpower for it. It's just that he doesn't. Thor does not stop. He does not consider. He does not ask sensible questions.
In Thor's mind there are only two categories: good and bad. Good things are always uncomplicatedly and unconditionally good and harmless, while bad things must be either avoided or smashed (preferably smashed).
This would be vexing at the best of times, but at present it is down right worrisome, because for some unfathomable reason, Thor has decided that snakes are good.
Now, as a general rule, Loki would not consider himself to be particularly afraid of snakes. In fact, he shares a measure of his brother's fascination for them: they are interesting and beautiful creatures. However, like all sane people in the nine realms, he has a healthy respect for the fact that many snakes are venomous and could kill him.
He knows that Thor knows this also. Thor has absorbed ever scrap of information on snakes he can get his hands on, as though he were starved for it. However, he has decided that snakes are good. He has got it into his head that he wants to touch snakes, to pick up every snake he comes across and admire it. Thor does not stop. He does not consider. He does not ask sensible questions. If he came across a venomous snake, he would not know the danger until the creature had put its fangs into him.
Mother has talked to Thor, of course. But Thor is simple, straightforward. He does not listen to talk, he only pays attention to action.
The solution is obvious.
It is up to Loki to act. Thor is overdue for a little persecution in any case.
Thor does not forgive him for nearly a month, which may be a record. However, the look on Thor's face as Loki transformed might almost have been worth it on its own.
Most importantly, though Thor takes more care around snakes. He may consider snakes to be good, but he definitely considers Loki's tricks to be bad, and Loki hopes he has the sense to realize that getting stabbed is bad also, though one can never tell with Thor. Venomous snakes may be of little consequence, but if the snake could be Loki with a knife, Thor may take the time to think before touching it.