We had the same issue. To resolve it we eliminated the table from the MemberMouse Login Page and modified the remaining code to fit on a phone as follows.
<div class="mm-login">[MM_Form_Message type='error']
<p style="font-weight:bold; font-size:125%;">Username</p>
<p style="font-weight:bold; font-size:125%;">Password</p>
[MM_Form_Button type='login' label='Login']<br><br>
[MM_Form_Field name='rememberMe' label='Remember me']<br><br>
<a class="mm-forgot-password" href="[MM_CorePage_Link type='forgotPassword']">Forgot Password</a>
Thank you for this suggestion! This feature request or some aspect of the functionality suggested is currently being reviewed for possible inclusion in a future update. If there is additional information, this post will be updated to reflect it.