Well, it’s not the first time Spike’s been the Joker.

Ooooh, what a clever joke for the comic relief! I’m kidding by the way!

@Sjogre  
The wild card is also the most useful card in games they’re legal in.

@Background Pony #1525  
He can fit in a set with other villains.

Actually, some of the symbolism of the Joker fits Spike, especially if you link the Joker to the Fool from Tarot.
 
Even aside from comparisons to the Fool, the Joker is often either the top trump or a wildcard, not a place of dishonor.
 
More importantly, there is only one Joker in a deck and isn’t part of any of the suites, like how Spike is the only dragon in Equestria and doesn’t belong to any of the three tribes of ponies.
 
Even if you aren’t looking at symbolism, Spike’s uniqueness means he’s unlikely to be chosen for any card that’s a set of four.

why they didn’t made Discord the joker instead of Spike?

@PonyPon  
It’s not like, you know, he’s exactly a popular character. I’m shocked that the SDCC exclusive includes him.

@Itsthinking  
Somewhat.

@PonyPon  
Would it surprise you if I said I fully expected this when I bought this deck?

Not what I expected.

Look at the bright side, Spike. You’re now the most sought after item in Jokers Wild video poker.