Is pH 2.1 a strong or weak acid?

It's considered a dilute acid, not a weak acid. That term has a very specific meaning.

A strong acid is one that ionizes (many chemistry texts use the Word "dissociate") completely in water, and a weak acid is one that doesn't completely ionize. Hydrochloric acid is a "strong" acid; hydrofluoric acid is a "weak" acid even though it's more corrosive than hydrochloric acid is.

So...you could take hydrochloric acid (a strong acid) and mix it with a lot of water, get it up to pH 6, and still have a strong acid.