Yes, street performers can be good, because there is nothing inherently better about someone's performance just because it is indoor. Street performances are sometimes better, because the performers know that they are performing to an audience that is close up. Street performers have no room for error. Sometimes, people are very talented and the street performances are great.
Those who are born performers need to perform somewhere, and it often takes more than talent to get oneself onto a major stage or into a major venue. So we see some of the most creative people honing their craft on the streets and we should support that with viewing and with money.
Street performances can be good. Street performances offer people on the move in daily life some form of quick entertainment usually for free unless they want to tip. It can bring people together and depending on the performance it can provide some cheap entertainment and shake up an rather boring routine.
The only thing required to have a quality street performance is an entertainer with talent. Therefore, I believe it is very possible to have a good street performance, or even an excellent one. Street performers succeed when they please their audience and while I'm sure they are far between, there are some out there.
Street performers, who are people who sing, dance, and otherwise entertain in exchange for donations, can definitely be talented. People who street perform have been seen competing, and doing quite well, in reality shows, such as So You Think You Can Dance. Believe it or not, Benjamin Franklin was even a known street performer!