I'm planning on buying a behringer u phoria UMC22 interface in order to connect my guitar to my PC to emulate effects and amps.
Seems a reasonable plan - though you are likely to get more latency going in and out of a PC than you would with a multi-effect unit (meaning that the sound coming out will be slightly delayed). If you're getting the interface only to produce guitar effects, and you don't need all the flexibility you get with PC-based virtual effects, you might want to consider what a cheap multi effect unit will give you - which would also give you freedom to practice away from the computer.
The amp has a 6.5mm guitar input jack and a Mp3 line in 3.5mm jack
Is it bad to use the amp as the speakers in this case? Can either components get damaged or result in bad sound quality?
You're unlikely to damage anything. The best sound quality is likely to come from plugging in through the 3.5mm jack, probably bypassing the high-gain amplification that you would normally need (because in this case, the interface is doing that job for you). For listening to complete songs, this is unlikely to sound great because of the limited range of the speaker - but for just monitoring your guitar it's likely to be fine.
Also the interface has stereo output with L/R jacks and I would only be able to connect one into the amp input jack
It looks like what you need is a cable that will go from a 3.5mm stereo jack to two 6.3mm (1/4") mono jacks.