My corn snake is pure muscle. Some corn snakes get a bit of 'junk in the trunk', but even though she is 'thicker', she definitely has muscle definition when she moves. I take her out regularly, let her climb and use her muscles.
I have a theory that some snakes, depending on genetics and whatnot, can have a lack of muscle definition due to their keeping. If they are forever kept in a smaller enclosure, limited things to climb on, limited interaction and 'coming out' to climb on people, they could develop a lack of muscle tone. This could also contribute to egg-bound females (since muscles are what move everything out when a female is laying or giving birth). I'd want my females to be fit and firm if they are laying.
I can't speak to this specific snake, as I can't see or handle her, but if she feels mushy she could be lacking muscle definition. Or there could be another cause that I am not experienced enough to comment on.
However, I can see how different animals would have different 'feels'. Also depends on how tense or relaxed she is as well.