Any fitment other than perpendicular to the exhaust tube, and with the sensor standing upwards, is going to have a worse effect than that. If you've got limited space, you can mount the sensor sideways, but the closer it is to horizontal, the worse it will clean out internal deposits. Gravity will hold them there, and if the sensor doesn't get hot enough, they won't burn off.
The tip of the sensor is made specifically to get a good diffusion(mixing of gases), when in the exhaust stream. If it's not in the stream, it will react much much slower, and possibly less accurate.
Mounting it wrong may give you a leaner(or richer) running system, but not by a safe amount, a fixed amount, or a known amount. E.g. if it runs slightly leaner at idle, it may run far too lean at cruise speeds.
Instructions usually explain how the sensor must be mounted, and how far from the engine. Respect these instructions, or you'll get an unpredictable unstable sensor output, that can develop worse over time. If you want to play with the mixture, rather tweak the sensor's output than it's location. And even that required skill, experience, and knowhow. I wouldn't recommend it. O2 sensors and their dependents are far more complicated than analog MAF or MAP or TPS sensors.