Is it acceptable to ask a SGM or 1sgt to be your mentor?

A mentor is an interpersonal relationship. While it's not inappropriate to ask a SGM to be a mentor, it will be more beneficial for you to have someone closer to your grade. Most of the relevant advice that you need will come from someone who recently passed through your same footsteps. Right now I think it would benefit you more to have a mentor who can give you exact instructions on how to accel in your current position and set yourself up for your first promotion to an NCO. A SGM may be able to give you general advice, but semi-centralized promotions have changed so much over the last decade that they won't be all that familiar with the ins and outs of the process.
So, yes it's definitely OK to ask them to be a mentor, but also, you'll still need a mentor closer to your own career progression.

PFC Tademy, your 1SG and SGM are already considered mentors as they mentor all ranks to include officers. They are extremely busy at times but are normally happy to help a young Private and provide personal guidance and mentorship and suggest possible ways to become a successful Soldier and future NCO. All NCOs and Officers are inherent mentors to the Soldiers which they lead, but to get the best benefit from a mentor I would suggest keeping the level down to someone you may see everyday as they will have the best knowledge of your performance and possible places you can improve. Your Team Leader will already be mentoring you through monthly counselings and daily guidance and leadership. Your Squad Leader should also be providing you with daily guidance and suggestions for improvement and encouraging the behaviors which are in keeping with the Army Values. Your Platoon Sergeant may not see you on a daily basis based on your MOS I would guess he may see you sporadically sometimes, but would interact with you and know the kind of Soldier you are. Those closest to you are your best bet and can provide the best bang for the buck so to speak. If the 1SG or SGM do talk to you and give you some guidance definitely follow it, they did not get to that position by chance they listened to their leaders and they know where you are in your career at this point and want to see you succeed.

If you want your 1SG or SGM to be your mentor then go ask them. A mentor is chosen by an individual and typically someone you look up to and truly respect. If those leaders between this Soldier and the 1SG or SGM are not what he wants to be like, then he should go to someone that is. If lower level leaders get offended because someone “jumps the chain” to get a good mentor, maybe they need to take a hard look at what types of values and characteristics they are putting out there for others to see. Believe me when I say that this 1SG and SGM will know how to properly navigate this relationship.
On the flip side, I do agree that if you have a leader more junior that you look up to, it would probably be more beneficial to you because he/she has been where you are more recently, but it doesn’t mean that the more senior NCO’s can’t still be great mentors for you.