Playing smash in general is useful, but just don't get too hung up on particulars while instead worrying about fundamentals. Things like having tight control of your movement, precise spacing, quick execution on frame traps, etc. will be universally worthwhile in every game. Things like being able to accurately execute the Ice Climber's chaingrabs are going to apply less, but of course, all skill has some transfer given that even if you're learning execution of a technique that simply won't be present, at least you're getting accustomed to learning tech skill.
For what it's worth, I found playing Melee before Brawl's release quite productive. It actually didn't even feel like I was improving with Melee, but as soon as Brawl hit, my relative skill level versus everyone around me just skyrocketed. I don't think the effect would have been nearly as dramatic had I not been playing.