Depends on what you want out of the game. CP is a function of attack, defense and HP of the species and its specific IVs of that pokemon.
However, CP on its own is useless. Eg. Slaking has the highest non-Mega CP in the game at 5K+ and is a piece of shit because it has a horrible fast attack.
This game is horrible at telling you whats good but generally high atk bulky pokemon tend to do best (since there no speed stat).
If you're looking to play ideally then dont power anything up unless its a good species AND has good IVs (High 3 star to 4 star).
Order of importance: Species > Moveset within that species > IVs
Since startdust is so hard to accumulate dont be afraid of keep high cp/level pokemon with bad IVs for general use while saving your dust for the best of the best. Realistically if something is a high 3 star its good enough but a lot time you will find a better one before you get enough candy to power it up to lvl 40 (let alone XL candy for lvl 50)
In short: dont power up that jumpluff or anything that isnt the best in its type (usuallu legendaries, megas, dragons/pseudolegendaries) or do it anyways because theyre cute. Stardust is not finite it just takes a long time to accumulate.