No, it's not wrong. It depends on the context. What is the connection between what they were doing before you got there, and your getting there? An isolated sentence doesn't usually give enough context to decide whether the past perfect is warranted.
"I promised to paint the room for him, but he was painting the room before I got there." The past continuous is right. The past perfect would be wrong in the second clause, though you could use it in the first.
"He had been painting the room before I got there, and the place really smelled." Here, there is a logical connection that works with the past perfect. The place smelled because he had been painting.
"He was painting/had painted/had been painting the room before I got there." There's no indication here that any specific tense needs to be used. There is simply not enough context. This is what your example is like.
