Ambrose is the most over guy in the company right now....but I don't think he's being held back or hampered in anyway with his Lesnar feud. To me, it's just building up even up more of a reason to like him.
Where they take him after WM is what concerns me as the logical (and easy) move would be to have Reigns win the title; become "The Face" and then Ambrose turn to halfway heel to become his chief rival. That doesn't help either as its shoehorning Reigns into a role that the fans don't want him in and not maximizing value on Ambroses' popularity. Next years storyline will also be complicated with the return of Rollins and the continuation of Lesnar as a constant, potential Number One contender to the title. Not even counting ANY of the other talent (Owens/Styles/Balor/Wyatt)....that gives you 4 legitmate, quality (at this point, anyone saying Reigns doesn't deliever in his matches is just hating) rivals for the title.
To that, I'd build The Legend of Ambrose up even more by having him beat Lesnar at WM. That gives Ambrose a legit claim to the Number One Contender mantle. If you have Reigns go over HHH....you can tweak Reigns to be a smug Champion, who recognizes Ambrose as his rival and abandons The Brotherhood.....or have HHH go over Reigns, have Ambrose win the belt in the weeks after WM and then have Reigns run a "I hate you Dean...you stole what was supposed to be my glory" angle.
One thing they should do regardless is give Lesnar the MIB. The idea of him legitmately being to cash in whenever would bolster ANY Championship Match.