Yes I agree a bit with this op. "MOB" as a kind of marketing concept is an odd thing. As pp are saying, it isn't exactly the same as a guest, so you wouldn't wear it to another wedding, and it is unlikely you would feel comfortable reusing a full-on MOB outfit for more than one of your dc.
For that reason, I think start with a) the style of the actual wedding at hand and b) what suits you, and forget about the MOB vibe.
I think for a fancy London hotel or evening reception that dress might be a little casual, but for a country wedding in the day it has a rather nice romantic feel - not too "don't forget to look at me, the sexy MOB" (and believe me I've seen a few of them!), but equally not too unfeminine.
But I would be guided by your DD. Ultimately if you and she are happy, I don't think it is "inappropriate." I do like the pale Phase 8 one another pp has just posted and which looks perhaps a little more summery and special occasionish?