why not ils sont?
des larmes is a modified noun, so you use c'est.
So, if we were leaving out the modifier 'of joy' and just saying 'they are tears', would it be 'ils sont des larmes'?
I think the modifier is "des" in this sentence. Ils sont larmes de joie. Ce sont des larmes de joie. Can we have an amen from a moderator?
There are....will be a better English.