I think when I say "trivial foo" I almost always mean "initial or terminal object in the category of foos" (but not necessarily a zero object, as many familiar categories don't have zero objects). Let's go through all of your examples to verify this:

the trivial group, ring, vector space, module over a ring, graph, category, Boolean algebra, group representation

Terminal objects.

homomorphism from one object to another

Depending on the objects, there many not be any such morphisms. I guess you refer to zero morphisms. When they exist in a category $C$, I think they should be something like the initial object in the category of functors $C^{op} \times C \to \text{Set}$ and extranatural transformations.

topology on a set

Which topology is the trivial topology? Either the discrete or the indiscrete topology could qualify here (the terminal and initial objects respectively in the poset of topologies on a set, ordered by inclusion).

measure

Initial object in the poset of measures, ordered by pointwise inequality.

The trivial automorphism of your favorite big object.

Initial object in the category of pairs (a group $G$, a homomorphism $G \to \text{Aut}(X)$).

Counterxample: The trivial graph is not final (or initial) in Graph.

This is not true. With the correct definition of the category of graphs, the trivial graph is the terminal object.

The only example you give that doesn't easily fit into the above classification is

knot

but in this case knots don't obviously form a category so one has to appeal to other notions here. For example, the unknot (I don't like the term "trivial knot" because it sounds like it could refer to the empty link) is the identity element with respect to knot sum, and initial and terminal objects are identity elements with respect to coproduct and product, respectively, so there's certainly a familial resemblance here. Trivial automorphisms can also fit here.

The categorical structure that knots do fit into is called the tangle category. In this category knots are morphisms instead of objects and I don't see a clear categorical description of the unknot in this context. I would avoid the term "trivial knot" for this reason but would be happy with the term "trivial link" to mean the empty link.

I think nearly everything I call trivial is the initial or terminal object in some category (but not necessarily a zero object, as many familiar categories don't have zero objects). Let's go through all of your examples to verify this:

the trivial group, ring, vector space, module over a ring, graph, category, Boolean algebra, group representation

Terminal objects. (You claim that the trivial graph is a counterexample, but with the correct definition of the category of graphs this is not true.)

homomorphism from one object to another

Depending on the objects, there many not be any such morphisms. I guess you refer to zero morphisms. When they exist in a category $C$, I think they should be something like the initial object in the category of functors $C^{op} \times C \to \text{Set}$ and extranatural transformations.

topology on a set

Which topology is the trivial topology? Either the discrete or the indiscrete topology could qualify here (the terminal and initial objects respectively in the poset of topologies on a set, ordered by inclusion).

measure

Initial object in the poset of measures, ordered by pointwise inequality.

The trivial automorphism of your favorite big object.

Initial object in the category of pairs (a group $G$, a homomorphism $G \to \text{Aut}(X)$).

The only example you give that doesn't easily fit into the above classification is

knot

but in this case knots don't obviously form a category so one has to appeal to other notions here. For example, the unknot (I don't like the term "trivial knot" because it sounds like it could refer to the empty link) is the identity element with respect to knot sum, and initial and terminal objects are identity elements with respect to coproduct and product, respectively, so there's certainly a familial resemblance here. Trivial automorphisms can also fit here.

One categorical structure that knots fit into is called the tangle category. In this category knots are morphisms instead of objects and I don't see a clear categorical description of the unknot in this context. I would avoid the term "trivial knot" for this reason but would be happy with the term "trivial link" to mean the empty link.