I think that's a really great point that you made here, and one I agree with! Identifying as polyamorous doesn't mean you are automatically consenting to any and all forms of a polyamorous relationship. It's true, too, that even in polyamorous relationships consent is important; in fact, I'd say that to be polyamorous in a healthy way it's important to be
extra thoughtful about consent and how you communicate and plan around it.
It isn't okay for your girlfriend to be pressuring you to be okay with her dating her crush (or anyone!) right now. If you have reservations about this, then it's important that you feel free to share them and that she listen and take them seriously. Also, has your friend/your gf's crush shown any interest in dating your girlfriend? It isn't clear to me if the two of them have talked about this possibility at all. Regardless, it just isn't fair, kind, or good polyamory practice for your girlfriend to dismiss any concerns casually, or to say that just because you're polyamorous it means you
must be okay with another relationship.
Also, I want to highlight what you said above because I think it's very important:
Im just a little scared to tell her how she hurt me and set those boundaries because I’m scared she’s going to tell me I’m not being considerate of her grief, which is what’s she’s been doing lately. Whenever I tell her something that’s been bothering me, she kinda tells me that I’m not considering her grief and it makes me feel like she just wants me to be okay with everything that’s happening and just blame the grief for her behavior and not take it personally.
As Sam said, setting boundaries or asking your girlfriend to treat you with respect in no way means you're disrespecting her grieving process. You can support a grieving partner by giving them some time and space to process their grief, and certainly it can be kind to understand that they may not be at their best while holding those painful feelings. However, it isn't cruel or unfeeling to insist on being treated well even while your girlfriend is grieving. She's still responsible for her own behavior during this difficult time, and it isn't acceptable for her to treat you badly or make bad faith arguments and then blame it on the grief she's feeling.
Of course she's not at her best right now; it sounds like her mother's death was very hard on her. But it still sounds like she's using that as an excuse for some behavior that isn't kind or loving on her part, and that's really sad and frustrating to hear. It sounds like you're already worried about expressing some basic needs and boundaries for fear of being ignored and dismissed, and that's just not a healthy position to be in, in a relationship.
I think this is something you do need to address with her, but I also think it's possible that she'll be unkind or dismissive when you do. But it isn't okay for her to treat you like this and keep using her grief as an excuse. Also: if she's grieving enough that it's difficult for her to treat you, her existing partner, well, then adding another partner is going to be a pretty bad idea for everyone involved, even if it's something you were okay with.