MACBA can be intimidating, but in my experience (as an older skater without a ton of talent), everybody was indifferent-to-friendly. I typically went a bit earlier in the day so it was wasn't totally mobbed.
And don't forget the Barcelona is crammed with amazing spots that are much less crowded than MACBA. Universidad ledges are right near MACBA. The vibes at Parel-lel were great (not sure how skateable that spot is today). The W curb was a really good spot for beginners or a mellower (but still lively) session. There are banks everywhere and several DIY parks.
The general rule for skating anywhere apply to BCN as well: if you're cool, they're cool. So just skate, be friendly, and try not to get in the way if somebody is filming a line.
Have fun!