Stargirl is a blaster with high Power Damage scaling - in fact, if she could land her skills while her target is affected by a max rank Gravity Well, her full combo has a Power Damage scaling of 425%, which is the highest of all blasters (Catwoman can theoretically do more damage with her ultimate, but over a far longer time). On the other hand, she can't burst as quickly as Sinestro. Stargirl lacks any movement abilities other than her passive, and is the only blaster to not have any form of hard crowd control (stun, fear, etc.) Fittingly, her suggested Stolen Power (other than Stargirl's Consume) is Super Strength. Due to this, she is vulnerable to being burst down by assassins, particularly those who has an ability to interrupt Energy Conversion's channel, such as Nightmare Robin.
On the other hand, the huge shield from Energy Conversion makes it very difficult for those who can't interrupt it to kill her if she survives the initial burst. Its cripple also prevents movement abilities, preventing Catwoman from interrupting it with Acrobat -> Cat's Paw. Its short range also encourages defensive usage.
The two parts of her passive are separate - Cosmic Blast will do additional damage over time regardless of whether she has recently taken damage.
Cosmic Blast's damage over time from her passive calculates its damage by the targets' health before the main damage of Cosmic Blast, and will not take any damage they take during the damage over time in account.
Cosmic Blast is a pass-through skillshot with relatively low cost that deals good damage as well as additional damage over time which scales to the enemy's current health, making it an excellent poke.
Shooting Star's first hit is slow and does not deal any damage, but it is targeted. The following 4 small stars are fast, deals a high amount of damage combined and has a longer range, but they are automatic skillshots that can be blocked by drones or dodged, especially when fired perpendicularly to the target's movement.
Gravity Well (slow) and Energy Conversion (cripple) could both help land Shooting Stars.
If the target dies before the all the small stars are fired, the ones that aren't fired yet would disappear, although the ones that have already been fired would continue forward until they hit a target up to a very long range.
The remaining stars would also not be fired if you lost sight of the target. X-Ray Vision can counter this problem.
Since the first hit does no damage, it is possible to mark a target with it under an enemy turret before backing out of turret range and let the small stars automatically be fired at the target without drawing turret fire.
Gravity Well has a fairly long travel time and a small area, so it can be somewhat difficult to hit opponents with it, although it would make it easier for Cosmic Blast and Shooting Stars to hit, as well as providing a noticeable damage boost. It is easier to use defensively.
While her three basic skills all have similarly decent ranges, Energy Conversion's range is very short for a blaster. However, it is Stargirl's only guaranteed source of damage (point-and-click and not blocked by other targets). It could be interrupted by hard crowd controls, so be careful of them, although its Cripple should prevent interrupts that are movement abilities from being used, such as Shazam's ultimate, as well as indirectly Catwoman's knock down from Cat's Paw, as she needs to use Acrobat first to grant it a knock down, although she could use Acrobat before being crippled by Energy Conversion.