Learn Spanish (Latin America): El mago fue patético; es más, arruinó el truco. - The magician was pathetic; what's more, he ruined the trick.

Es más acts as an argumentative escalator. It doesn't just add a random detail; it introduces a stronger, more extreme piece of evidence to prove the first point. The speaker is saying the magician was pathetic, and as the ultimate proof, he actually ruined the trick.

Could I use encima or por si fuera poco in this sentence instead?

Yes, both would work well since they are also used for escalation. Encima (on top of that) often carries a stronger tone of annoyance or complaint. Por si fuera poco (as if that weren't enough) emphasizes that the situation was already bad before this final point was added. Es más is slightly more neutral, meaning what's more or in fact.

Why is there a semicolon before and a comma after es más?

When linking two independent clauses (phrases that could stand alone as complete sentences) with a connector like es más, Spanish punctuation rules usually require a semicolon or a period before the connector and a comma immediately after it.

Why use the preterite fue instead of the imperfect era for 'was pathetic'? Isn't it a description?

While background descriptions in the past usually take the imperfect (era), the preterite (fue) is used here to evaluate a completed event as a single, finished whole. The speaker is passing a final judgment on the magician's performance at that specific show, rather than describing what his act was generally like over an ongoing period of time.