Just for me, personally ... I'd have to say 6lbs or better would be the cutoff. I've caught 4lb SMB, and like you ... they mostly came when Crappie fishing. :D
My reasoning is simply this ... the 4lb fish have come too easily & frequently, and the state record is 8.5lbs +/- ... so, a fish that falls in between those weights, or heavier, is where I'd make my minimum cutoff line. And while a 4 or 5lb fish is certainly a nice SMB, if I'm going to fork out the $$ for a mount, it's going to have to be a seriously nice fish :p
I'm still looking for a wall hanger Crappie ... but, 3lb Crappie just don't come all that often ... LOL !! I've experienced 2.5 - 2.75lb Crappie, and hung but lost Crappie that I believe would have been 3lbs (twice :o ) ... so I set 3lbs as the minimum "wall hanger" weight.
Here's a story, to illustrate my thinking on "trophy" fish ....
As a much younger man, I was heavy into Bass fishing. My goal was to put a 5lb LMB on my wall. I managed to hook & lose several fish that would have easily gone 6lbs + ... but, managed to eventually catch some that exceeded my 5lb minimum. The problem was, I caught a 5lb & a 6lb LMB, all on the same trip :p By that time, I'd changed my thinking a little on what I considered to be a real "wall hanger" fish. It wasn't so much a specific size, anymore, as it was "one that I felt I'd probably never surpass". Fast forward a few years ... and I hook & land a 7lb LMB. The lakes I was fishing at that time, were not giving up many fish of that size ... so I opted to call this 7lb fish my personal best, and have it mounted.
The only other fish I've had mounted (also caught Crappie fishing ) was a 12lb Tiger Muskie. The only reasons I had it mounted, was because I figured I'd never catch a bigger one, and I caught it on UL equipment & 4lb line (no steel leader) ... and I figured I'd never duplicate such a feat. :p
Neither of my mounted fish are "exceptional" fish, in their own rights (for their respective species) ... they're just "personal exceptions", fish that are my personal best & unlikely to be surpassed (by me). That's what has become my interpretation of a "trophy" fish. At my age, it's not so much about the size/weight of the fish, that makes it a trophy worthy of having mounted ... it's the story behind the catching of the fish.
... luck2ya ... cp