As a character I think it's both kind of obvious and not really interesting at least to me. Minecraft is a huge franchise, of course, and has a very distinct visual style, but while Steve is iconic (and they're adding some other Minecraft characters, presumably as skins) he's also a total cipher and I have no attachment to him. Maybe people who grew up with Minecraft do? There are books, adventure games, and other media starring him so maybe if you've dipped into that you see more in him than I do.
Of course with Smash it's all about how he plays, and we don't know that yet. It could be interesting, given that it's Minecraft, and it should at least look fun. I think it's always neat to add more visual variety to that game, which does the clash of styles thing really well.
What's most interesting to me is what this says about the relationship between Nintendo and Microsoft. Two of the Fighters Pass additions to the game have been iconic MS owned characters, but Minecraft is sort of treated as its own separate property (as shown by the fact that it's on PlayStation too) so maybe this doesn't mean anything?
Still, I find it interesting whenever the two companies do something together. The big elephant in the room is whether it's possible that Project XCloud Streaming could ever appear on Switch. I doubt it, but I'm sure Microsoft really wants it to happen and I wonder if they're playing nice with Nintendo in the hopes that some day they can make that deal.