My opponent screams after every touch, is that allowed?

Mostly yes, and it drives new fencers crazy. The celebratory yell after a touch is deep fencing culture, you'll hear it from eight-year-olds to Olympic finals. It's adrenaline, self-hype, and (between experienced fencers) a little psychological warfare. It is not against the rules by itself.

Where the line actually is: yelling AT the referee to sell a call can draw a warning; screaming directly in an opponent's face, taunting, or excessive celebration after every single touch can be carded as unsportsmanlike or 'disturbing order', referees have discretion and use it, especially in youth events where they'll quietly tell a kid to dial it down. A fencer who yells before the referee has even called the action is also making the ref's job harder, which never helps their cause.

How to handle it from the other side of the strip: expect it, so it stops being surprising; use their scream time as YOUR reset time (they're performing; you're planning the next touch); and never match energy you don't feel, composure reads as confidence, and nothing deflates a screamer like an opponent who calmly wins the next three touches.

If it genuinely crosses into intimidation at a youth event, a parent's move is to speak to their own coach, not the referee or the other kid, coaches handle strip-side culture problems constantly and know the local officials.