Learn Spanish (Latin America): ¡Qué tendencia más difícil!

You are right that más usually means "more". However, in exclamations following the pattern ¡Qué + noun + más + adjective + !, it acts as an intensifier. Instead of making it a comparison, it emphasizes the adjective, translating to "What a difficult trend!"

Could I use tan instead of más in this sentence?

Yes, absolutely! In this specific exclamatory structure, más and tan are completely interchangeable. Saying ¡Qué tendencia tan difícil! means exactly the same thing and is just as common.

Why isn't there a word for "a" before tendencia?

In English, we say "What a difficult trend!", but Spanish does not use the indefinite article (un or una) after ¡Qué!. You just attach qué directly to the noun. Adding una here would sound incorrect to a native speaker.

Does tendencia just refer to a popular "trend", or can it mean a personal "tendency"?

It can mean both! Depending on the context, tendencia can refer to a cultural or market "trend" (like a trend on social media), or a personal "tendency" or inclination to do something.