My MBP shipped with Lion but I actually 'upgraded' to Mountain Lion via the Developer program before it was officially released. I haven't had a problem with it at all... no kernel panics, no apps 'freezing', nothing.
Your problem could either be hardware or software related. You might want to try a not-so-clean installation of ML, running the Apple Hardware Test, etc.
For an in-depth look as to why your computer might be slow, take a look at ds store's excellent user tip - Why is my computer slow?
thanks for replying to my post earlier,but i want you to know that i deleted mountain lion and im back with lion and everything is working as normal as before ,im happy now real happy offcourse i got a option to download another copy for mountian lion,did'nt do it but i dont know in the future i may but scared cause i feel mountian lion is not for me,thanks for ur support my friend.(for the record i did all the possible test n all were good but still the computer was freezing n stuff so i jus got rid of it.)