I approached my internship advisor for an LOR while applying to Masters. She said she would not be able to provide a strong one but a good one based on an internship that lasted for two months. I approached her because she holds a PhD from a good university in the USA and she's published papers in top-tier conferences and journals. Will it adversely affect my application? Will it hold as much weight as a strong one because it's she who's recommending it?
Will it adversely affect my application?
No, but it probably won’t help it much either.
Will it hold as much weight as a strong one because it's she who's recommending it?
Keep in mind what “strong recommendation” actually means. It’s not about the forcefulness of the language; it’s about the quality of evidence that the letter provides for your success in the graduate program. The writer’s reputation/credibility is definitely part of that, but that informs how credible I should find the evidence they write about, not just how credible I should find their unjustified opinion.
Compelling evidence from someone relatively unknown has more weight than a bare thumps-up from a superstar.
This is probably subjective because it will depend on the person reading the letter, and whether he/she knows the person who wrote the letter personally.
The way I see it: if the letter does not clearly state or imply anything negative it will just be seen as a good recommendation. If however the letter mentions (or implies!) negative points you may be better off with a good or excellent letter from a less well-known scientist.