Is a spirit's influence considered a mind effecting ability? Would a creature that was immune to mind effecting abilities be immune to a spirit's influence from a poor pact?
No. You're effectively allowing an alien being into your very soul, and its by influencing your soul (not necessarily your mind) that spirits can overall impact your abilities and actions.
Its more akin to possession (which, as you'll note from its inclusion in Occult Adventures) is not a mind-affecting effect. That said, things that protect you from possession don't protect you from a spirit's influence because you essentially surrendered any ability to defend yourself from the spirit's influence as part of the pactmaking process. That's why its called a "poor pact," after all. ;-)
Second question, assuming that the influence is mind effectong, could a character that was immune to the influence still make pacts?
See above. If you could somehow be immune to a spirit's influence, you'd effectively gain no power from that spirit because its power is a direct result of that influence. A "good pact" is simply managing to convince the spirit not to try to exert its will upon your actions in favor of simply "enjoying whatever ride it gets," while a "poor pact" is agreeing to allow the spirit a measure of control which it will often try to abuse spectacularly.