It's not, it's pretty good. The GC version might be better, but the 3DS version is by no means bad. Hell, it got better reviews than the GC original.
Then again, a launch title never receives the best reviews.
Luigi's Mansion is an interesting concept and game idea, but it is nowhere near as good as it is thought to be. I like the gameplay, but it does lack quite a few elements to make it even close to a timeless classic or essential buy for anyone interested in the Mario franchise.
As for the topic itself, I'll be waiting for the results before I give my opinion on the issue. The name of the developer is one thing, but the actual output of an acquisition is another. Hopefully the results should be visible in the next few years anyhow.