Yes, the objection that the discovery requests "seeks a legal conclusion from a lay witness" could be a valid objection (depending upon how the discovery request is worded, of course).
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney.
Just to add onto the answer already provided, however, simply stating an objection does not trump the duty to answer the request.
Not sure from your question, but there are plenty of times when attorneys will state blanket objections and fail to satisfactorily answer the question (especially against in pro per parties). If you have a situation that the request is being objected to and not answered, you need to follow up and get the question answered. Basically, in the discovery arena, the only valid reason to not answer a question is based upon a privilege (exceptions apply, but "legal conclusion" falls outside of this).
Of course, a lot of this is dependent upon the type of discovery request you seek an answer for--written discovery (which type), oral deposition, etc. You might try and seek an attorney for limited representation in your case for discovery issues only. Best of luck.
Yes, it may be a legitimate objection. But written discovery (as opposed to deposition discovery) may inquire about the application of facts to law, because it is written with the assistance of counsel. So whether the objection is well-founded depends on the exact question in the context of the facts of the case.
This response is provided as general information only. It is not intended to be legal advice. Legal advice must be based on the exact facts of the particular situation, and by necessity this forum is not appropriate for discussion of specific, exact facts. Contact a lawyer for more specific advice. My answer to your question on AVVO does not create an attorney-client relationship.