@bilalayub919! Brother, which brand of rims does Suzuki use on GS150 as stock? AFAIK, SOMO rims are the worse quality rims (after cheap local ones), and at Rs.2400, you purchased it very expensive. The 1.6x17 (rear CG125) SOMO rim barely costs Rs.800. How can this cost 3x more money? :S (I am sorry i am not aware of the price difference between different sized rims as i never inquired about it from the market).
Also, buying better quality rims is just HALF the story. Find a person who can align your new rims 100% perfect. If you are in Lahore, there is a famous guy in McLeod Road who can do that. I can't remember his name, but if you require, i'll ask tomorrow from my own wheel balancing guy (who knows of his work).
Changing just the rear rim won't do. You will have to change BOTH of them. Also check the tyre pressure, and inflate tyres exactly how your owner manual (or rear chain cover) mentions.
Lastly, PBC (Pakistan Bikers Club) said that new GS150s are coming with unaligned frames. If all else fails, Please contact PBC to see if your bike really suffer from that problem.