A different shapeshifter I would believe could shift an optic nerve– whatever female changeling said “to become a thing is to know a thing”…. and so much of my existence as a human is based in how my body reacts??? you can’t know a thing if you’re not shapeshifting the organs and nervous response system etc

Buuut like in The Begotten baby changelings can still sense their surroundings when they haven’t shifted into other forms…

So basically Odo is a lazy or not talented shapeshifter who relies on basic changeling senses instead of shifting organs to do the dirty work, so when those senses are overwhelmed via rain/stimuli he doesn’t know how to handle it.