Depth doesn't determine how good a game is at any level (else most sports really wouldn't be that "good" and games like Dota would be the epitome of design, neither of which is objectively true), though it certainly helps keep people's attention.
That said, Smash4's depth is partially limited by rulesets. Custom moves add these options that so many people like, yet due to their hidden status, many players shun an excellent source of variety. Stage selection also limits depth. Strategies change significantly between hazards, walkoffs, flatplats, and so on. And yet ancient customs dictate the trend and lock us out of certain varieties of play just because they have elements that were considered "uncompetitive" in past iterations of the series. Heck, random and min-max based as it is, Equipment could add extra depth if we were willing to go to such extents.
There's a lot of base-level depth, and there's room for more if we want to adopt rules that let us use it.
Also, as someone studying Japanese, I always love it when people use "Yomi" as their definition of depth. Since in Japanese, that means "Reading". Which Smash4 has plenty of.