I think it's looks OK, though a bit mushy/blurry in places. What's the texture resolution? UV layout? Topology?
It also seems to be the same basic pattern all over the whole rock, which doesn't happen much in real life. Lichens tend to congregate in certain areas, dirt in others.
A rock in isolation isn't much to look at, sticking it in an environment helps with judging whether it fits or not.
The lighting is also not very helpful, front lighting is bland and uninteresting.
Well i hate when i see polys in the contour of a model. You could chamfer those sharp edges or subdivide it a bit and then optimise in order to add more polys in the places with more tension. I also hate uv seams, blending different grunge maps together even though they have nothing to do with each other. Also, i hate it when i look at a model and cant tell if its a mountain side or a pebble. The scale of the features is very important and your model is confusing. At first glance i thought your model was medium to large scale but then i noticed the lychen which is similar to what you would see up close on a rock so the scale has to be small to medium then. Find good reference and dont forget to take into account the scale of those references.
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Also, i hate it when i look at a model and cant tell if its a mountain side or a pebble. The scale of the features is very important and your model is confusing. At first glance i thought your model was medium to large scale but then i noticed the lychen which is similar to what you would see up close on a rock so the scale has to be small to medium then.
Find good reference and dont forget to take into account the scale of those references.