I assume that you are an SFDC user?
I'm afraid that the mobile phone field on the user object (lead owner) is not currently part of defualt the mapping. If you can find a way in SFDC to populate a custom field with the lead owner's mobile phone number them Marketo will pick that up no problem.
I remember I tried to do this in SFDC before and ran into roadblock, but I see that they have since made owner (user) fields available for use in formulas, so this should make it much easier.
https://success.salesforce.com/ideaView?id=08730000000BrqaAAC
You would just create a formula field on the lead with this formula:
Owner:User.MobilePhone
But there a some
gotchas with formula fields that you need to be aware of.