No, it will be not practical
Main reason for that will be no the concerns you mention, but because of difference between civilization.
One barely can reach the orbit of the own planet, but other managed to make the interstellar journey, with military goals I guess. Sure it is in the case when they (aliens) do not have some strange deviation, assuming that sending interstellar probe like voyager will be enough to defeat planet civilization.
More general it is expected that they can plan ahead.
More wise to invest efforts into ground to orbit missiles, and use warheads not to create debris field, but as missile warheads potentially it will have more impact. Or if you like that debris idea - create it where you need it and When you need it - to fight particular target.
Debris is just not efficient
It might be considered as a danger for satellites, but we talking about constructions you might break with a screwdriver(by stabbing, multiple times, or once if you more careful stabber).
Our biggest construction in space(earth orbit particular) is ISS with 400tonne of mass. Would it be capable of interstellar travel - I would discourage to use it as the military vessel to defeat a planet based civilization at the stone age level of development.
And the level of threat which is dangerous for ISS probably is not significant for possible construction of offender side. Bigger construction is less percentage by mass will be that layer which protects the construction, and more efficient this layer will be.
Another moment of inefficiency - we tended to use lowest orbits which are suitable for our tasks. We do that because of serious limitations of our launch technologies. Not the case with aliens because they are already in space - they can choose any orbit from above, not from below as we do. So to have hope to affect them you should make a thick layer of debris, up to GEO - and you still can't be sure it will be enough, even in the case that debris would be a treat for their crafts.
Not viable, not practical, not smart, not efficient.
For humans, the planet is their stronghold, and hope, hope for the aliens will engage them on the planet, and to be not completely destroyed(in a military sense) until aliens begin ground-planetary operation, and pray they will.
Aliens in that situation more than capable of destroying planet population, and actually planet it self - just because one reason, they will have so much time as they wish or need to make suitable space infrastructure for that task. Our current abilities to project power is limited to low orbits.
If creating debris field is considered as the option - it means humans have time to prepare, my recommendation is - go for the moon, but do not stop for 50years, go farther create you own space infrastructure, this is their only hope to be able to defend themselves and pray to make it in time.
Minimum requirements are to be able to project power up to 5 a.u., prevent aliens to use Jupiter as the source of matter and prevent them from using the sun as an energy source if they need it and preventing them from preventing us to use it, because we definitely need that source.
Easiest way for alien to defeat your humans is to block sunlight to planet, by placing primitive construction in L1