Funny, while I see a lot of similarities between Merrill and Tali in the whole "Cute, sweet, and awkwardly funny" vein, I still think Merrill has more in common with Liara. Or maybe its that Merrill is more a Liara/Tali mix.
In ME1&2 Tali isn't as dependent on Shepard as Merrill is with Hawke. To some degree all companions rely on Shepard but Tali always carried herself with more confidence. She knows she's the best engineer around, she knows she's lethal with a shotgun (more importantly Grunt and Garrus both know she's deadly with a shotgun), and she wants to help Shepard fight the geth (eventually Saren and the reapers).
Merrill's story arc comes off a bit more like Liara, confident in her abilities but really she'd just rather be working/researching on her one goal (Liara - Protheans, Merrill - the Eluvian) which makes them outsiders to their own people. They both have mother issues, and while it's possible to do Noveria without Liara, both can potentially face off against their mothers (or mother like figure in Merrill's case), both of whom are controlled by another evil force. Friendmance with Merrill is also similar to Liara in that they both see their hero as a big force and they are simply drawn in. (Rivalmance has a more unique flavor to it.)
I'm sure I can draw up more similarities between all three, but beyond the cute factor I think Merrill is more like Liara than Tali. I just happened to find Merrill nowhere near as annoying as Liara! (Annoying award goes to Anders).
Full Disclosure: I honestly believe the love story between Shepard and Tali is one of the best written in video game history. I also believe that besides Shepard, Tali is the most important companion, regardless of love interest or not. Without her data, ME1 goes no where, unlike every other character who is expendable (Kaidan and Ashley were both bla, Garrus and Wrex, while awesome, are optional, and Liara was just kinda annoying and did nothing more than the green Asari on Feros could have done). In ME2 she becomes important to Shepard, either as trusted friend or love interest, and later as a peace maker between Quarian and Geth (should you pursue those options in game).