I have been thinking about the expected effect of mutation accumulation - and I think there would be de-differentiation/ loss of specialized adaptations.
These would affect general intelligence 'g' (because g is a fitness measure), and adaptive social functions (because these are subtle/ advanced adaptations which are damaged by even slight illness, intoxication or any functional brain impairment).
I think evidence consistent with both lowered intelligence and also impaired adaptive social functioning can be observed in the report of Mouse Utopia.
The reduced fertility in Mouse Utopia is perhaps also related to impaired drive/ motivation - as well as ineffective drive/ motivation (due to loss of functional adaptations).
I general, I think loss of adaptive functionality is what should be looked-for with mutation accumulation (i.e. adaptive behaviours knocked-out or damaged or distorted), rather than weird new behaviours - and particularly loss of functionality in:
1. The social domain - first subtle, then gross impairments of adaptive social interactions
2. The sexual domain - first subtle, then gross impairments of adaptive sexual interactions
...bearing in mind that 'adaptive' means tending to enhance reproductive success.
I suggest social and sexual functioning, since these are the areas which I think are the most sensitive to brain impairments; at least that seems to be the situation in neurological and psychiatric disease.
My observation has been that when there is almost any significant degree of neurological or psychiatric disease, even the slightest; social and sexual domain functioning can usually be detected as having been impaired, by those who best knew the patient before he suffered illness.