Well you can be too fat, as the dress only ever went up to a size 18.
I do think it is pertinent to consider our age and size when buying clothing (I am a size 20) and it is worth considering that just because they make it in your size doesn't mean something will be 'flattering' to wear.
If you genuinely have no fucks to give about what people think about your clothing choices then obviously go for it and wear what you like, I think most people want to wear something that brings out their best features so taking age and size into account is perfectly normal.
That said, I do think that style of dress would be suitable for most ages/sizes.