No.
A CR (Challenge Rating) is calculated assuming all of the creature's powers are crowded into a single encounter. Player characters get to spread those powers out over the course of an entire campaign... Allowing them to utilize the powers to greater effect.
The number you're looking for is the monster's Level Adjustment. The character's total level ends up as: Level Adjustment + Monster Hit Dice + Levels in a character class.
More information can be found in the SRD: Monsters As Races
Acrobatic Strike is a poorly written feat that does not make any sense.
It requires that you Tumble to avoid an Attack of Opportunity, and gives you a bonus against the foe who otherwise would have made an Attack of Opportunity.
The problem with this is that you do not get to make a Tumble check when an opponent announces his intent to make an Attack of Opportunity; you have to do it before you move. And once you’ve done so, you do not provoke at all, and no one is making any Attacks of Opportunity. The wording of Acrobatic Strike states that you only get the bonus against an opponent from whom you’ve avoided an Attack of Opportunity, but if you never provoked he never got the option to choose whether or not to make one in the first place.
And fixes like assuming that you get the bonus against anyone who you would have provoked from had you not made the check are also awkward. What if a character had Reach you were unaware of, and you would have provoked an Attack of Opportunity but had not realized it? What if you would have provoked, but the target had no Attacks of Opportunity remaining? Or if you’d observed him having made one Attack of Opportunity, but he has Combat Reflexes?
Overall, the feat is poorly designed, and ultimately the effect, however you rule it, is fairly weak. I recommend just not bothering with it at all.
But you can Tumble on a 5-ft. Step, as it is movement, and as long as you hit DC 15 (unless you take a −10 penalty to move at full speed, though, you don’t actually go anywhere), you avoid Attacks of Opportunity. Not that 5-ft. Steps provoke anyway, but that does not seem relevant; Tumble doesn’t have any restriction of that sort, nor does Acrobatic Strike.
Best Answer
Confirm the Fluff with the DM
The only thing important about the exalted feat Nymph's Kiss (BE 44) is what it is (an exalted feat) and what it does (the feat's Benefit). Everything else about that feat is purely descriptive. The Player's Handbook says
The feat Nymph's Kiss has no prerequisites.
The DM can always house rule prerequisites for the feat, but if he doesn't, once the player's received the DM's permission and the player's character has taken the feat, the feat's there. Only if the character commits an evil act or is "in areas where magic is suppressed or negated (such as an antimagic field)" (PH 180) does the feat disappear.
Evil Acts
As the feat Nymph's Kiss is an exalted feat,
Thus, if the character were "trying to dupe someone into buying a cursed item" (DMG 277), which is an evil act, all the character's exalted feats' benefits would be lost until he atoned.
Areas of Suppressed or Negated Magic
Strangely, a character loses all exalted feats in areas where magic doesn't function:
Luckily, the character with the feat Nymph's Kiss probably spent the skill points he gained, so he needn't subtract those when he enters such an area, but it's something to keep in mind if those feats are prerequisites.
Really, Talk to the DM
The DM must approve the feat, and he must be on board with your character taking a feat that mandates your character be good or lose the feat. If the DM's used to putting devastating moral quandaries in front of the characters, forcing them to constantly choose the lesser of several evils, the character will be hard to play and might not fit the campaign. That's a thing. Talk to the DM. Absolutely.
But only the DM can house rule that your character must do something to keep his feat. That's his call... even if the feat has no prerequisites.