There's nothing at all wrong with it. You are part of the lab. It is a proposal to fund the lab (and you, by extension). And really, I've always felt my lab to be really a team-based atmosphere. Sure, the advisor was in charge and had final say and everyone has their individual projects, but they all should be pulling in the same direction.
Furthermore, as a graduate student, the proposal will either (a) be based significantly on the work you have done, or (b) be work you will do. Or (perhaps most likely) both. In any case, you have perspectives and ideas that even the PI does not have, so you should contribute whenever possible.
I was lucky to be involved in several proposals while in graduate school (Including NSF, ARPA-E, so fairly major ones). Not to say I "enjoyed" proposal writing, but it was/is a skill that as served me well in my post-academia career, and really has allowed my to contribute in my current position. And, it's worth noting that the technical writing skills of proposal writing translate fairly well to report writing, and really paper writing. Honestly, reading some of the journal papers out there, I wish more students got more involved in technical writing opportunities earlier in their careers.