Not really so long as they are an effective therapist.

Some years back, my therapist was my pastor. His full-time, true calling in life was as a pastor. He didn't become a therapist (yes, he was a fully trained and licensed therapist) because he didn't make enough money as a pastor, so it isn't quite what you are asking, but his therapy work was definitely only part-time and not his first calling. He was an excellent pastor AND an excellent therapist.

Do you assume if therapy isn't their main profession or calling that they are only doing therapy work to supplement their income? Because I don't think that is always (or perhaps even usually) the case. Or, are you asking because you have seen the money motivator as a problem with your personal therapist?

I think even if money was a motivator for doing therapy part-time, that wouldn't negate their ability to be good at what they do with their clients.