Any Graph is a Model (! or ?) I know this could be considered a pointless question. However despite I am quite convinced that any possible model (i.e. UML, SysML, natural language, math, etc.) can be defined by means of a graph I do not know if there is a way to formally prove there exist an isomorphism between the set of Models and the set of Graphs.

I guess one should first define what a model is and I guess a possible answer is that a model can be considered as a sentence of the language defined by its meta-model. Thus models are languages. The question is therefore if there is an isomorphism between the set of languages and the set of graphs or, less formally, if it is provable that any possible language can be represented by means of a graph. I guess that the Chomsky works on languages could help but I can not find a reference on the Internet of such demonstration.
If you look at things from a model-theoretic perspective, the basic ingredients you have are atoms, relations, and functions. Atoms could be modelled using nodes of the graph, and binary relations and single argument functions could be modelled using graphs. If you are willing to allow hyper-edges in your graph, then the remainder of the relations and functions can be modelled. 

I don't think that there is anything deep to this observation.