Is it appropriate to reach out to my selected major's dean at the schools I apply to and is it a means of showing demonstrated interest at the school?
A few questions to consider before you do that:
Why are you reaching out to the Dean of Biology (or any other course)? Do you have specific questions for them that cannot do not appear on the college website? Writing to say "I am aware you do research, but I would like to know if undergrad students are permitted to work on r-CRISPR-1089 with a view to synthesizing it, or whether that is left to graduate students," is different from writing to say "I love your Biology program. How tasking is the course load?"
One of these things can be easily found on the website and by mailing current students. Another cannot be easily found, and I suspect the Dean may appreciate the fact that you already have a clear idea of the sort of research you want to engage in.
Another question: how do they respond to students who have done this in the past? You can get a feel of this by asking the admissions officers or current students. Do the Deans welcome emails from prospective students, or do they outrightly not reply?
You must bear in mind that deans are very busy people, so you must ensure that whatever email you send is highly crucial, and the information you seek would not ordinarily be found on the school's website.
You could always contact students who school there from any one of CollegeVine's webinars.