I would jump to the same conclusions too - is this a sign of a lack of professionalism. Even if it had been the staff member assigned to this I would have at least found a good envelope. Alas, as most offices no longer have a typewriter, you see a lot of handwritten envelopes these days. Still, you would think something would be said about the illegibility. Of course, I am assuming an assistant is responsible for this.

I think a better indication of the professionalism is what is contained within the envelope. How then is the correspondence itself? Is it well written?