Well, it was Walt's decision that the outside of the Mansion should be pristine. He was obsessive about keeping the parks looking like a showplace in every way. With that said, there is precedent for a more-weathered Mansion area. The grounds around the Tokyo version are overgrown, however it works better there because their mansion looks to be higher up on a hill. I'm just going by photos, so I could be wrong. Phantom Manor in DLP goes full-dilapidation. I really like the look of it. There are arguments on both sides, I guess. I think, though, with the rest of Liberty Square being so pristine, a run-down Mansion might really look out of place.
Indeed, the Disneyland Mansion was designed to be pristine for fear that even an intentionally dilapidated building would reflect poorly on his park. Disneyland was working against the public mentality that amusement parks were dirty, unkempt places, and Walt felt so strongly about presenting the opposite that it carried over even to a supposedly haunted building.
The same logic was applied to the design of the MK Haunted Mansion, despite its more gothic facade. Back around opening the Liberty Square grounds looked like an Estate, intricately manicured but curiously designed. It always took me back to the Vanderbilt Mansion here in New York, which always felt like it was beautifully kept by zany people. There was something otherworldly about it because the over-the-top house was maintained with such real-world care and precision. It seems the Tokyo Mansion was similar on it's opening, but I can't say for sure.
Phantom Manor had to eschew this logic by nature of it's placement in Europe - the park was designed to cater to an audience that might be coming from any country in Europe and may speak any number of languages, so the best way to communicate clearly was visually. "Haunted Mansion" didn't translate nicely into many European languages, but "Phantom Manor" says the same thing in a way that meets more languages in the middle. But better than that, you can tell just from looking that the spooky, old, "Psycho"-looking House on the Hill is exactly what it looks like, and you know before you even come close to the sign if that kind of ride is up your alley or not. Thankfully by the 90's Disney had cemented the idea that their parks were clean, well-kempt places, and could afford to subvert this idea in the name of storytelling for Phantom Manor.
It seems they felt the same way when Splash Mountain came to Tokyo Disneyland that year in 1992, which resulted in some queue changes to their Haunted Mansion - some new, crumbling crypts were added, and the lawn started to grow over . . . I believe that's also when their famous broken window was added, it definitely wasn't there from the beginning. The early 90's must have been a turning point in the mind of the company, because it wasn't long after that the MK Mansion followed a similar lead of permitting a more derelict look to set in with a toppled fountain and less meticulous landscaping.
To round this out with my own opinion, I enjoyed those old trees on the front lawn, but look forward to giving the Mansion a look-over without them there. I'm willing to accept the Liberty Square Mansion as "overgrown", but it should be manicured within reason instead of truly overgrown. Generally I don't like the deforestation happening at The Magic Kingdom, but these trees had been left to grow beyond reason. If we could just find a way to do away with that canopy I'd be thrilled, though I won't hold my breath for that one.
I first went in 1983 and I never caught even a tiny vision of the show building I'm thinking that you would have had to be pretty tall to see it even then. Now across the river, well, that is a different story in the 70's.
. We don't know. It's poor if WDW allows a sick tree to fall, and crush people - just cleaning up the mess can be serious work 
