Not everyone accepts random friend requests, sadly
I however don't really care either way, and accept almost any Smash friend request I've gotten (which has been quite a few). The problem with this though is I have a list of 40-something friends that I have no clue who they are. We don't really send messages or anything.
There have been one or two that has attempted to message me on Miiverse at one point asking to Smash, but unfortunately for them I pretty much never use Miiverse to begin with. Either way, even if you did have players accept your friend request the chances of being able to Smash with them again would be severely slim depending on if they use Miiverse any, if not at all as well as you actively sending them messages. Otherwise you shouldn't give your hopes up that much with the whole friend request thing to be honest.
If you really want to find Smash players that are good and very fun to play with, you should possibly try here instead.
There is an entire Thread dedicated to finding opponents and friends (for Wii U & 3DS) here. I've used the Wii U one frequently when the mood strikes me, and I've had tons of amazing matches from other members here. Everyone was either on my skill level or higher.
I only met one person here who I lagged with as well, while everyone else had pretty great connections. It's always worth a shot.
Opponent Finding Thread for Wii U
Opponent Finding Thread for 3DS