Actually, I believe there is a mention of Tessio at the wedding - there is a scene later in the book (right before he goes to travel with Michael and Tom to see Barzini) that he sees a young lady who happened to have danced with him (she was only 8 years old at the time) and she has fully matured as a beautiful woman.

However, I agree, Tessio is really more of the minor character than Clemenza.