There's more things to like about a character then, well, their character. You might not care, I don't expect you to - honestly I don't care about Steve at all myself - but there's a lot of different reasons to want someone then just their personality.
I think people would like Steve for one or both of two reasons.
1) They love Minecraft and want a character who represents the gameplay and feel of it.
2) They think his janky movements and art style are amusing or possibly even charming in their own weird way.
Both of these reasons are completely legitimate reasons to want a character. The former is why we have characters like Villager and the latter is why we have characters like Mr.Game&Watch and Wii Fit.
I think it's reasonable to
care about the character--not the sole reason, but a big reason. Otherwise lets have generic Fornite soldier.
1) There are other ways to represent franchises, and Minecraft's appeal comes more from the world rather than the characters. Even the Creeper is more iconic than Steve
2) Potential movesets matter more in a fighting game than purely aesthetics, and the off-the-wall characters have been iconic to Nintendo in particular (Game & Watch, Duck Hunt, Wii Fit). Picking a character purely for aesthetic appeal, not their abilities, is a better suited for games like Mario Party and Mario Kart (which is why we probably still don't have Waluigi)
My criteria for a good Smash character: 1) Moveset potential (the biggest factor) 2) Popularity/Relevancy 3) Significance with Nintendo