Kargil Plan on paper was good because it identified a weakness which if exploited properly and at proper time could have strategic results.
Mistakes:
1- There was no rationale for such out-of-blue provocation on diplomatic front so Kagril Op tried to drive its diplomatic justification from 'Its Mujahideen and not Army'. This took away a lot freedom of operation from PAF and PA. I mean how would you justify PAF and PA supporting war effort of Mujahideen? Rather they should have tried to lure Indians into some sort of aggressive action in Siachen in preceding months and then used it as pretext of Kargil OR if that was not possible then they should have just waited for the right time.
2- Under-estimating response of a powerful adversary e.g. IAF won't be involved. I mean why would India not call in Air force against so-called insurgents on its controlled territory, when its at risk of losing another important controlled territory(Siachen).
3- Not realizing that even if aim was to keep conflict limited, it would require reasonable conventional and non-conventional deterrent from other two services to prevent India from expanding the conflict. Late 90s was worst time for such action, we were under sanction for many years and air force was severely effected by sanctions. AM Kaisaer Tufail wrote that Indian Air activity was at minimum when F-16s were at station doing CAPs. But no other fighter was considered good enough for CAPs in that situation and since spares were being consumed eventually F-16 CAPs had to be abandoned. Imagine the disruptive effect round the clock PAF presence would have had on Indian air effort. Remember Kargil was all about surviving the summers while disrupting supplies to Siachen. So anything that bought you time was important. So here is how I imagine PAF to operate if it had Full Freedom of Op and was well-equipped:
i) Indian low altitude bombing were effectively catered for by ManPADs.
ii) India went to high altitude bombing which required recon to be accurate. PAF could off and on ambush these ground attack configured aircraft to disengage. This could be done by simply achieving radar lock and then maintaining presence in the region(disruptive effect).
iii) Next India to counter PAF ambushes starts sending escorts. PAF could try to lure Indian escorts to engage them and make them follow Pakistani fighters into Pakistani territory. Irrespective of the fact whether Pakistani or Indian aircraft got shot down in ensuing fight. It would give Pakistan the card to play on diplomatic front i.e. Look Indian Air Force violated our airspace and shot down our plane or got its aircraft shot down; so from now on we will not sit back and be more proactive. Obviously Indian would have made counter-claims but in fog of war facts are hard to verify. So all you needed was enough justification to carry out more aggressive air-to-air campaign. From here on wards, it would be which ever Air Force fought better air-to-air war. But the bottomline would have been that IAF would be forced to concentrate its effort on air-to-air campaign and disruption in anti-ground effort was all Pakistan needed.
Conclusion:
So Kagril Op was good at tactical level but its planners failed to realize the importance of right time, role of other services and diplomatic environment.
i) So yes Musharraf and his co-leagues made blunder in planning and execution.
ii) Nawaz made a blunder when he fell for flattery and self-serving presentation of the Army planners rather than listening to his Sec. Def. and go for an early abort.
Anyways, if Kagril was executed properly at the proper time; Pakistan may have achieved its eventual goal of gaining upper hand in Siachen but road to Siachen would still have been difficult, long and bloody as India would have done everything in capacity to retain its position in Siachen.