Ugh.
This is what I hate about modern wrestling, and particularly the build up to Wrestlemania. Nowadays we don't get any story. All the build up to a match needs is hype. They can just tell us the match is happening for five weeks in a row and expect that it's enough to make people want to see the match. There's no need to actually get people engaged in some kind of ongoing story. Nothing interesting has to happen. Nobody has to be emotionally invested at all. There doesn't have to be some kind of pay off to events that have happened over the last few months. The only thing that's important is that they announce the match a bunch of times, and let us know its happening. Basically the bare minimum.
Because after all, who wouldn't go out of their way to watch a match that they
know is happening? Such people clearly don't exist.
I'm not trying to say that I think Undertaker's segments would be amazing if he came back now to build to the match. I'm sure he'd be involved in more of the same old shit. But better something than nothing. It'd be nice to see what kind of relationship these two wrestlers have before they fight. To get a broader sense of what the issue is between them. To see if they have any kind of chemistry.
The only reason I can see to keep Undertaker off television based on the description above is if he's going to radically alter his appearance in some way that will make for a cool spectacle when he first walks down the ramp. And no, I don't mean another freaking haircut, although apparently some fans will pop for that.

So I would say a bad idea. I'm almost offended by the idea to be honest. Though probably just using Undertaker at all is a bad idea at this point. He's not in the best of shape these days and he's never been less relevant.