Link and Pikachu. It should be what draws people to Nintendo.

Are we talking Pikachu? I cant see Ash or the brand name being the mascot. You need a character from within the game to be a mascot and Mario is everything a mascot needs and should be.
Pokemon is popular but its popular as a whole while Mario Bros is popular due to its character Mario. I can see Pikachu being close.

Neither, Red was a more striking colour in the NES era imho, someone like Charmander would be

According to Smash Trolls: Goku.

This...also the anime series was quite popular in the late 90s. Pokemon had to arrive when it did on the GB in order to achieve its massive boom.

If you replaced Super Mario Bros with the first Pokémon games the industry wouldn't be where it is. Give props to Mario and a lesser extent Donkey Kong as they carried the industry out of the muck.

Ka-pi96 said: But... the NES kind of flopped in Europe. That means Pokemon probably would have flopped here too :O

According to whom?

Still Mario. It wouldn't have mattered if Pokemon was even more successful than it actually was in the 90s.
Mario is Miyamoto's baby. That's his Mickey Mouse while he's Walt Disney. At best they'd be co-Mascots. But they would still prioritize Mario in the advertising. 

My son likes Mario a whole lot more than Pikachu. So I say Mario. I know it's quite a small sample size, but whatever.

Compare all pre PS1 console numbers and you'll see NES numbers were what the entire European market was at the time in the 80s, NES was even the best selling gaming platform to ever release in Europe at the time so your logic is flawed.