As others have contributed, it is unlikely to become sponsored if you haven't shown good results in the past. However, having a sponsor is NOT equivalent to being a professional and there are probably many "bad poker players" who have sponsors.
For example, perhaps a celebrity is playing in some event. They might be sponsored just because of their famous status which has nothing to do with how good (or bad) they are at poker.
I feel like some sponsors know this and use the act of "sponsoring" as an ego boost to the victim player. By the player bragging they are "sponsored" it makes them feel like they "made it" in poker, but this might just be the sponsor taking advantage of their vulnerable confidence level.
Naturally, this isn't for every case (and maybe not that many). I still feel like some sponsors use this as a tool to get better deals with the player than perhaps they deserve. The humble player should realize that sponsorship isn't as huge of a deal as some might believe it is, but this is just my opinion and observation.