Some clinics have limits, however I was in the low 30s for BMI when we cycled and our FS did talk to me about weight and IVF success.

She was matter of fact but supportive, basically said every kilogram I lost would increase our chances. I lost about 4kg in the month before we started and we had success (we also had male factor issues)

I guess though that even if a clinic doesn't have a weight limit, the reality is that it's going to be a factor so worth losing anything you can, even a few kgs are going to be a bonus