The typical four-string bass might not be, but the bass itself in general can be a good solo instrument if you are good enough at it. Think about an 8-string bass 0r even 12-string bass, which allows you to reach notes that even guitars can reach! This allows for more showmanship and technical wizardry than if you’re playing a four-string. But that doesn’t mean you can’t play amazing solos on a 4-string, either. It all depends on your level of mastery and proficiency with the instrument. And if you are really good at the bass, then it can be fun to play solo!