Tiramisu help, why is it bumpy?

Question

as I was mixing in the mascarpone cheese, why did the mixture become bumpy instead of smooth?

Answer 1

the Mascarpone was too cold.  You should have let it come to room temp so it blended in easier.  You can overcome that too by beating the Mascapone with a mixer until it is smooth and then adding the other ingredients a little at time until it is incorporated.

Answer 2

Sounds like the custard broke, which happens from over  mixing it and/or it might have been too warm.